FROM -THE- LIBRARY- OF 

•WILLIAM -A  HILLEBRAND 


DYNAMO  LABORATORY 
OUTLINES 


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DYNAMO  LABORATORY 
OUTLINES 


FOR  STUDENTS  IN 

ELECTRICAL  ENGINEERING 


BY 

JOHN  FAY  WILSON,  B.  S.,  E.  E. 

DEPARTMENT   OF   ELECTRICAL   ENGINEERING, 
UNIVERSITY   OP   MICHIGAN. 


McGRAW-HILL  BOOK  COMPANY 
239  WEST  39TH  STREET,  NEW  YORK 

6  BOUVERIE  STREET,  LONDON,  E.  C. 

1913 


COPYRIGHT,  1913  BY  THE 
McGRAw-HiLL  BOOK  COMPANY 


THB.  MAPLE.  PRESS.  TORK. PA 


PREFACE 

The  preparation  of  these  Outlines  was  undertaken 
with  the  idea  of  supplying  a  laboratory  manual  of  limited 
scope  and  cost  yet  containing  such  material  as  would 
meet  the  requirements  of  electrical  engineering  courses 
generally.  To  this  end  a  study  was  made  of  all  available 
information  regarding  the  laboratory  work  of  a  large 
number  of  American  universities  and  technical  schools. 
It  was  found  impracticable  to  include,  in  this  volume, 
every  experiment  listed  by  these  schools  but  the  substance 
of  every  experiment  having  general  engineering  interest 
has,  the  writer  believes,  been  incorporated. 

While  the  writer  does  not  believe  in  " spoon  feeding" 
neither  can  he  subscribe  to  the  other  extreme  of  making 
the  student  an  independent  discoverer  of  the  facts  and 
principles  pertaining  to  the  laboratory  assignment. 
The  tourist,  visiting  Colorado,  saves  time  and  gains  more 
information  by  employing  a  competent  guide  than  by 
starting  out  alone  to  "discover"  Pike's  Peak.  The 
tourist,  however,  gains  little-  information  by  simply 
following  the  guide.  He  must  use  his  faculties  of  obser- 
vation. So  the  laboratory  student,  may  be  lead,  by 
means  of  a  proper  outline,  to  certain  experimental  facts 
which  he  should  connect  with  the  theory  as  developed 
in  the  class-room  or  by  outside  reading. 

These  Outlines,  therefore,  consist  of  short  but  explicit 
instructions  regarding  the  performance  of  the  experiment, 
and  conclude  with  a  list  of  questions  covering  both  the 
theory  and  the  practical  operation  of  the  apparatus 
studied.  (It  is  not  expected  that  the  questions  asked 
will  cover  all  phases  of  the  subject  that  may  arise  and 

v 

995899 


vi  PREFACE 

instructors  may  find  it  advantageous  to  ask  additional 
questions.) 

It  is  not  intended  that  the  order  in  which  experiments 
are  arranged  in  this  volume  should  indicate  the  order  in 
which  they  should  be  performed.  Neither  is  it  expected 
that  the  subject-matter  under  one  heading  should,  neces- 
sarily, be  covered  in  one  laboratory  period.  The  sub- 
divisions of  the  subjects  make  it  possible  to  omit  parts  of 
any  subject  where,  for  lack  of  time  or  any  other  reason, 
it  may  be  deemed  advisable. 

The  results  of  the  use  of  these  Outlines  in  the  writer's 
classes  at  the  University  of  Michigan  have  been  most 
satisfactory  but  corrections  or  suggestions  for  their 
improvement  will  be  gladly  received  from  any  one 
interested. 

The  thanks  of  the  writer  are  due  Prof.  Benj.  F.  Bailey, 
of  the  University  of  Michigan,  for  encouragement  in  the 
preparation  of  these  Outlines  and  for  suggestions  for 
their  improvement;  also  to  Mr.  Walter  M.  Rennie  and 
to  Mr.  G.  W.  Snedecor  for  reading  and  correcting  the 
manuscript. 

J.  F.  W. 

THE  UNIVERSITY  OF  MICHIGAN, 
January,  1913. 


CONTENTS 

PAGE 

GENERAL  INSTRUCTIONS 1 

Direct  Currents. 

1.  PRELIMINARY  STUDY 7 

2.  THE  SHUNT  GENERATOR 9 

Building  up. 
Characteristics. 
Components  of  voltage  drop. 
Armature  characteristic. 

3.  THE  SERIES  GENERATOR 12 

Building  up. 
Characteristics. 

4.  THE  COMPOUND  GENERATOR 14 

Building  up. 

Characteristics. 

Calculation  of  compounding. 

Determination  of  the  number  of  turns  in  the 

shunt  field  winding. 
Changing  the  degree  of  compounding. 

5.  CONDITIONS  AFFECTING  VOLTAGE 17 

Brush  position. 

Magnetic  leakage. 

Armature  reaction  and  armature  drop. 

Speed. 

Field  excitation. 

6.  PARALLEL  OPERATION  OF  GENERATORS 19 

Shunt  generators. 
Compound  generators. 

7.  THE  SHUNT  MOTOR 21 

Direction  of  rotation. 
Performance  curves  from  loading. 
Performance  curves  from  the  losses. 

8.  THE  SERIES  MOTOR 24 

9.  THE  COMPOUND  MOTOR 26 

10.  STATIC  TORQUE 27 

11.  CONDITIONS  AFFECTING  THE  SPEED  OF  A  MOTOR.    .     29 

12.  STRAY  POWER 29 

vii 


viii  CONTENTS 

PAGE 

13.  ARMATURE  MAGNETIZATION 36 

14.  MAGNETIZATION  CURVES 37 

At  no-load. 
At  full-load. 

15.  MAGNETIC  LEAKAGE 39 

16.  HYSTERESIS  LOOP  AND  LENGTH  OF  AIR  GAP  ....     41 

17.  FLUX  DISTRIBUTION  IN  THE  AIR  GAP 43 

Single-pilot  brush. 
Double-pilot  brush. 

18.  BOOSTER  ACTION 46 

Line  booster. 
Battery  booster. 

19.  HEAT  TEST 48 

20.  THREE-WIRE  SYSTEM 49 

21.  THE  MOTOR-GENERATOR  AND  THE  DYNAMOTOR     .    .     51 

22.  INSULATION  RESISTANCE 52 

23.  THE  VARIABLE  SPEED  MOTOR 54 

Alternating  Currents. 

1.  PROPERTIES  OF  A  CIRCUIT  CARRYING  AN  ALTERNAT- 
ING CURRENT 55 

Part  1. — Resistance  and  reactance  in  series. 
Part  2. — Resistance  and  reactance  in  parallel. 
Part  3. — Voltage  resonance. 
Part  4. — Current  resonance. 

2.  THE  ALTERNATING-CURRENT  GENERATOR 62 

The  saturation  curve. 
The  short-circuit  curve. 
Synchronous  reactance. 
Regulation. 
Efficiency. 

3.  PARALLEL  OPERATION  OF  ALTERNATORS 69 

4.  THE  SYNCHRONOUS  MOTOR 72 

Starting. 
V-curves. 
Clock  diagram. 
Circle  diagram. 
Efficiency. 

5.  THE  ROTARY  CONVERTER 76 

Starting. 
Compounding. 
V-curves. 
Efficiency. 


CONTENTS  ix 

PAGE 

6.  THE  INDUCTION  MOTOR      80 

Starting. 

Performance  curves. 

No-load  test. 

Blocked  rotor  test. 

Construction  of  the  circle  diagram. 

Power  factor. 

Slip. 

Efficiency. 

Maximum  output. 
Balancing. 
Cascade. 

Frequency  changer. 
Single  phase. 

7.  THE  INDUCTION  GENERATOR 89 

8.  THE  SINGLE-PHASE  COMMUTATING  MOTOR 91 

9.  THE  CONSTANT  POTENTIAL  TRANSFORMER 93 

The  losses. 

Efficiency. 

Regulation. 

Kapp's  diagram. 

Heat  test. 

Ultimate  temperature. 

Separation  of  the  iron  losses. 

10.  THE  AUTO-TRANSFORMER 101 

11.  TRANSFORMER  CONNECTIONS 102 

Single  phase. 
Polyphase. 

12.  IRON  LOSSES 109 

Apparatus. 
Steinmetz's  exponent. 
Steinmetz's  coefficient. 

13.  THE  CONSTANT  CURRENT  TRANSFORMER 114 

14.  THE  MERCURY  ARC  RECTIFIER 116 

15.  INSULATION  (BREAKDOWN)  TEST 118 

16.  WAVE  FORM 122 

APPENDIX.     POWER  MEASUREMENTS  AND  WATT-METER 

CONNECTIONS    .    .  .   126 


GENERAL  INSTRUCTIONS 


PROTECTION  OF  APPARATUS 


Any  electrical  apparatus  that  is  to  be  usecf  m-  experi- 
mental work  should  be  connected  to  the  supply  circuit 
by  means  of  a  double-pole  switch  and  protected  from 
excessive  currents  by  suitable  fuses  or  circuit  breakers. 
Time  and  trouble  will  be  saved  if  all  connections  are 
carefully  checked  before  closing  the  switch.  During  the 
early  part  of  the  experiment  it  is  well  to  keep  a  sharp 
lookout  for  trouble,  particularly  for  overheated  coils, 
rheostats  and  bearings. 

INSTRUMENTS 

Select  instruments  of  such  a  range  and  scale  division 
that  accurate  readings  may  be  obtained.  Ammeters  are 
to  be  connected  in  series  with  the  load,  voltmeters  in 
shunt  (parallel)  with  the  load.  A  wattmeter  is  a  com- 
bination of  ammeter  and  voltmeter  coils  acting  on  one 
moving  element.  Wattmeter  diagrams  and  connections 
will  be  found  at  the  end  of  this  volume. 

DATA 

All  observations  must  be  accurately  and  neatly  tabu- 
lated. Sample  calculations  should  be  included  in  the 
written  report  so  that  both  the  method  and  the  arith- 
metical result  may  be  readily  checked. 

1 


2  DYNAMO  LABORATORY  OUTLINES 

CURVES 

Curves  should  be  drawn  in  ink  on  cross-section  paper. 
Draw  smooth  curves  through  as  many  of  the  experi- 
mental points  as  possible.  It  is  not  expected  that  a 
curve  (which  is  simply  a  method  of  averaging  results) 
will  pass  through  every  experimental  point  determined. 
Bfeth  the  looks  and  the  value  of  a  curve  depend  much  on 
:  „;  ;; ;  -the  s^lectitori  of  the  scales  to  which  the  curve  is  to  be 
drawn. 


QUESTIONS 

At  the  end  of  each  experiment  will  be  found  a  list  of 
questions.  Each  of  these  is  to  be  answered  briefly  but 
explicitly. 

REFERENCES 

No  attempt  has  been  made  to  explain  the  theory  in- 
volved in  the  experiment.  The  student  is  expected  to 
look  this  up  for  himself.  The  more  thoroughly  he  does 
this,  the  greater  will  be  the  benefit  derived  from  the 
work.  As  an  aid  to  this  end,  a  few  selected  references 
are  added  at  the  end  of  each  experiment.  Other  refer- 
ences may  readily  be  found  in  any  good  engineering 
library  and  the  student  is  urged  to  form  the  habit  of  look- 
ing up  the  views  of  different  writers  on  any  subject 
under  consideration. 

ACCIDENTS 

The  treatment  for  electric  shock  is  so  simple  and  failure 
of  its  immediate  application  so  fatal,  that  a  lack  of 
knowledge  of  the  treatment,  particularly  among  electrical 
men,  is  little  short  of  criminal. 


GENERAL  INSTRUCTIONS  3 

The  immediate  effect,  when  the  body  comes  in  contact 
with  an  electric  conductor  of  any  considerable  voltage, 
is  a  suspension  of  the  act  of  respiration.  Artificial  respi- 
ration must  be  established  immediately  or  death  is  inevi- 
table. The  fact  that  immediate  action  must  be  taken  if 
the  life  of  the  patient  is  to  be  saved  cannot  be  too  strongly 
impressed.  Life  or  death  is  a  question  of  a  very  few 
seconds  but  only  after  the  heart  ceases  to  act  is  the  case 
hopeless  and  of  this  a  qualified  physician  is  the  only  per- 
.son  competent  to  judge. 


RULES  RECOMMENDED  BY 

Commission   on    Resuscitation 
from  Electric  Shock 

REPRESENTING 

The  American  Medical  Association 

The  National  Electric  Light  Association 

The  American  Institute  of  Electrical  Engineers 

DR.  W.  B.  CANNON,  Chairman  DR.  GEORGE  W.  CRILE 

Professor    of     Physiology,    Harvard  Professor  of  Surgery,  Western  Reserve 

University  University 

DR.  YANDELL  HENDERSON  MR.  W.  C.  L.  EGLIN 

Professor  of  Physiology,  Yale  Univer-  Past-President,      National      Electric 

sity  Light  Association 

DR.  S.  J.  MELTZER  DR.  A.  E.  KENNELLY 

Head  of  Department  of   Physiology  Professor  of  Electrical   Engineering, 
and  Pharmacology,    Rockefeller    In-  Harvard  University 
stitute  for  Medical  Reseach  DR    ELJJJU  THOMSON 
DR.  EDW.  ANTHONY  SPITZKA  Electrician,    General    Electric    Corn- 
Director    and    Professor    of    General  pan2/ 

Anatomy,  Daniel  Baugh  Institute  of  MR.  W.  D.  WEAVER,  Secretary 

Anatomy,  Jefferson   Medical  College  Editor,  Electrical     World 


Copyright,  1912,  by 
NATIONAL  ELECTRIC  LIGHT  ASSOCIATION 

Follow  these  instructions  even  if  victim  appears  dead. 

I.  IMMEDIATELY  BREAK  THE  CIRCUIT 

With  a  single  quick  motion,  free  the  victim  from  the  current. 
Use  any  dry  non-conductor  (clothing)  rope,  board,  to  move  either 
the  victim  or  the  wire.  Beware  of  using  metal  or  any  moist 
material.  While  freeing  the  victim  from  the  live  conductor  have 
every  effort  also  made  to  shut  off  the  current  quickly. 

4 


RESUSCITATION 


II.  INSTANTLY  ATTEND  TO  THE  VICTIM'S  BREATHING 

1.  As  soon  as  the  victim  is  clear  of  the  conductor,  rapidly  feel 
with  your  finger  in  his  mouth  and  throat  and  remove  any  foreign 
body  (tobacco,  false  teeth,  etc.).  Then  begin  artificial  respiration 
at  once.  Do  not  stop  to  loosen  the  victim's  clothing  now;  every 
moment  of  delay  is  serious.  Proceed  as  follows: 

(a)  Lay  the  subject  on  his  belly,  with  his  arms  extended  as 
straight  forward  as  possible  and  with  face  to  one  side,  so  that 
nose  and  mouth  are  free  for  breathing  (see  Fig.  1).  Let  an 
assistant  draw  forward  the  subject's  tongue. 


FIG.  1. — Inspiration;  pressure  off. 


(6)  Kneel  straddling  the  subject's  thighs,  and  facing  his  head; 
rest  the  palms  of  your  hands  on  the  loins  (on  the  muscles  of  the 
small  of  the  back),  with  fingers  spread  over  the  lowest  ribs,  as 
in  Fig.  1. 

(c)  With  arms  held  straight,  swing  forward  slowly  so  that  the 
weight  of  your  body  is  gradually,  but  not  violently,  brought  to 
bear  upon  the  subject  (see  Fig.  2).     This  act  should  take  from 
two  to  three  seconds. 

(d)  Then  immediately  swing  backward  so  as  to  remove  the 
pressure,  thus  returning  to  the  position  shown  in  Fig.  1. 

(e)  Repeat  deliberately  twelve  to  fifteen  times  a  minute  the 
swinging  forward  and  back — a  complete  respiration  in  four  or 
five  seconds. 

(/)  As  soon  as  this  artificial  respiration  has  been  started,  and 


6  DYNAMO  LABORATORY  OUTLINES 

while  it  is  being  continued,  an  assistant  should  loosen  any  tight 
clothing  about  the  subject's  neck,  chest,  or  waist. 

2.  Continue  the  artificial  respiration  (if  necessary,  two  hours  or 
longer),  without  interruption,  until  natural  breathing  is  restored, 
or  until  a  physician  arrives.  If  natural  breathing  stops  after  being 
restored,  use  artificial  respiration  again. 


FIG.  2. — Expiration;  pressure  on. 

3.  Do  not  give  any  liquid  by  mouth  until  the  subject  is  fully 
conscious. 

Give  the  subject  fresh  air,  but  keep  him  warm. 

III.  SEND  FOR  NEAREST  DOCTOR  AS  SOON  AS 
ACCIDENT  IS  DISCOVERED 


DIRECT    CURRENTS 


PRELIMINARY    STUDY 

The  object  of  this  experiment  is  to  study  the  structural 
details  of  the  different  types  of  dynamo  and  the  con- 
struction and  operation  of  motor-starting  rheostats. 

1.  Study    the    electrical    and    the    mechanical    con- 
struction of  the  following  types  of  dynamo : 

(a)  shunt. 
(6)  series, 
(c)  compound. 

2.  Draw  a  diagram  of  the  electrical  circuits  of  each 
of  the  machines  studied. 

3.  Make  a  sketch  for  each  machine  studied  showing 
each  of  the  following  parts  in  its  relation  to  the  others, 
explain  its  function,  its  construction  and  state  of  what 
material  it  is  made: 

(a)  armature  core. 

(6)  field  core. 

(c)  yoke. 

(d)  commutator. 

(e)  brushes. 

4.  Draw   a   diagram   of  the   electrical   circuits   of   a 
motor-starting  rheostat  having  " no-load"   and   " over- 
load" releases  and  indicate  how  it  is  connected  to  the 
circuit  of  a  shunt  motor. 

7 


8  DYNAMO  LABORATORY  OUTLINES  [l 

5.  Connect  up  a  shunt  motor  with  an  ammeter  in  the 
armature  circuit,  move  the  rheostat  handle  to  the  first 
stud  and  note  the  maximum  deflection  of  the  ammeter 
pointer. 

Note  the  maximum  deflection  for  each  stud  as  the 
handle  is  moved  slowly  over  its  range. 

6.  Repeat    (5)    for   alternate   studs,    beginning   with 
the  first. 

7.  Plot   diagrams  using  as  ordinates  the  maximum 
current  indications  of  (5)  and  (6)  and  rheostat  studs  as 
abscissa. 

8.  Determine  the  resistance  of 

(a)  the  armature  winding. 

(b)  the  armature  circuit. 

(c)  the  field  winding. 

(d)  the  field  circuit. 

9.  Explain 

(a)  why  the  armature  core  is  built  up  of  thin  sheets 
(laminations)  instead  of  being  made  in  one  solid 
piece. 

(b)  why  it  is  not  so  necessary  that  the  field  cores 
be  laminated. 

(c)  the  difference  between  the  " short-shunt"  and 
the  " long-shunt"  compound  dynamo. 

(d)  the  reason  for  using  a  motor-starting  rheostat. 

(e)  the  function  of  the  " no-load"  release  and  how 
it  operates. 

(/)    the  function  of  the  " over-load"  release  and  how 

it  operates. 
(g)  what  mechanism  is  often  used  in  place  of  the 

" over-load"  release. 
(h)  the    difference    between    a    "motor"    and     a 

"generator." 


2]  DIRECT  CURRENTS  9 

(i)  by  means  of  diagrams,  the  determination  of 
the  resistance  of  the  armature  winding  and  of 
the  field  winding. 

REFERENCES 

Franklin  &  Esty,  Direct  Currents,  Chap.  2  and  pp.  112-116. 

Karapetoff,  Exp.  Elec.  Eng.,  Chap.  15-16. 

Smith,  Testing  Dynamos  &  Motors,  pp.  81,  95,  110. 


THE  SHUNT  GENERATOR 

The  object  of  this  experiment  is  to  study  the  shunt 
dynamo  as  a  generator. 

1.  Building  Up. — (a)  Connect  a  shunt  dynamo  as  in 
Fig.  1,  drive  it  at  constant  speed  and  measure  the 
voltage 


FIG.  1. 

(1)  with  the  field  circuit  open. 

(2)  with  the  field  circuit  closed. 

(3)  with  the  field  terminals  reversed. 

(6)  Drive  the  dynamo  at  the  same  speed,  but  in  the 

opposite  direction,  and  repeat  (a), 
(c)   Explain 

(1)  how  a  voltage  is  generated  when  the  field 
circuit  is  open. 


10 


DYNAMO  LABORATORY  OUTLINES 


[2 


(2)  why,  with  a  given  field  connection,  a  shunt 
generator  will  build  up  when  rotated  in  one 
direction  but  will  not  build  up  when  rotated 
in  the  other  direction. 

(3)  why,   with  a  given  direction  of  rotation,   a 
shunt  generator  will  build  up  when  the  field 
current  flows  in  one  direction  but  will   not 
build  up  when  the  field  current  flows  in  the 
opposite  direction. 

2.  Characteristics. — (a)  Connect  as  in  Fig.  2,  drive  at 
the  rated  speed  with  such  excitation  as  will  give  rated 
voltage  at  full-load  and  determine  the  following  for 
current  outputs  up  to  150  per  cent,  of  the  rated  load: 


Load 


FIG.  2. 

(1)  field  amperes. 

(2)  terminal  voltage. 

(6)  Determine  the  armature  resistance. 

(c)  Define 

(1)  total  characteristic. 

(2)  external  characteristic. 

(3)  regulation. 

(d)  Using   current   as   abscissa  and  e.m.f.  as  ordi- 
nates,  plot 

(1)  the  external  characteristic. 

(2)  the  internal  characteristic. 


2] 


DIRECT  CURRENTS 


11 


(3)  the  armature  drop. 

(4)  the  field  current. 

(e)   Calculate  the  voltage  regulation. 
(/)  Explain,  by  means  of  a  diagram,  the  determina- 
tion of  the  armature  resistance. 

3.  Components  of  Voltage  Drop. — As  the  load  on  a 
shunt  generator  increases  the  voltage  decreases  (speed 
and  field  resistance  remaining  constant).  This  voltage 
drop  is  due  to 

(1)  the  resistance  of  the  armature  circuit. 

(2)  armature  reaction. 

(3)  decreased  field  current. 


o  Amperes 

FIG.  3. 

(a)  From  the  data  taken  for  the  characteristic  (or 
from  the  curve  itself)  determine  the  voltage  drop 
for  different  values  of  armature  current  and  plot 
as  oa  in  Fig.  3. 

(6)  Keeping  the  field  current  constant  (separate 
excitation),  determine  the  voltage  drop  for 
several  values  of  armature  current  and  plot  as 
ob. 

(c)   Calculate  the  resistance  drop  and  plot  as  oc. 


12  DYNAMO  LABORATOKY  OUTLINES  [3 

Then,  for  the  armature  current,  od,  ab  is  the  drop  due 
to  the  decreased  field  current,  be  is  that  due  to  armature 
reaction  and  cd  is  that  due  to  the  resistance  of  the  arma- 
ture circuit. 

(d)  Explain 

(1)  how  the  resistance  drop  varies. 

(2)  armature  reaction. 

(3)  why  the  field  current  decreases  as  the  load 
increases. 

4.  Armature  Characteristic. — The  armature  character- 
istic shows  the  relation  between  field  excitation  and 
armature  current,  the  speed  and  the  voltage  being 
constant. 

(a)  Connect  as  in  Fig.  2,  run  at  rated  speed  and 
voltage,  and  record  field  amperes  up  to  150  per 
cent,  of  the  rated  output. 

(6)  Plot  a  curve  using  field  current  as  ordinates  and 
armature  current  as  abscissa. 

(c)   Explain 

(1)  why  the  field  current  increases  with  the  load. 

(2)  why  the  rate  of  increase  in  the  field  current 
is  greater  for  large  armature  currents  than  for 
small. 

REFERENCES 

Franklin  &  Esty,  Direct  Currents,  Chap.  4-5. 
Smith,  Testing  of  Dynamos  &  Motors.  Chap.  10. 
Karapetoff,  Exp.  Elec.  Eng.,  Chap,  16-17. 
Bedell,  D.  C.  &  A.  C.  Testing,  Chap.  2. 

3 

THE  SERIES  GENERATOR 

The  object  of  this  experiment  is  to  study  the  action  of 
the  series  dynamo  when  operated  as  a  generator. 


3J 


DIRECT  CURRENTS 


13 


1.  Building  Up. — The  series  generator  cannot  build 
up  unless  the  external  circuit  is  closed  because  the  ex- 
ternal current  is  also  the  field  current  and  no  current  can 
flow  on  open  circuit. 

2.  Characteristics. — (a)    Connect   as   in   Fig.   4   and 
measure  the  voltage  for  outputs  up  to  150  per  cent,  of 
the  rated  load. 

(b)  Determine 

(1)  the  armature  resistance. 

(2)  the  field  resistance. 


0000000 


FIG.  4. 

3.  Plot 

(a)  the  external  characteristic. 

(b)  the  internal  characte'ristic. 

(c)  the  RI  drop  of  the  armature. 

(d)  the  RI  drop  of  the  field. 

4.  Explain. 

(a)  for  what  purpose  the  series  generator  is  used 
commercially. 

(b)  why  the  characteristic  curve  bends  to  the  right 
as  the  load  increases. 

REFERENCES 

Franklin  &  Esty,  Direct  Currents,  Chap.  3. 
Smith,  Testing  of  Dynamos  &  Motors,  Chap.  7. 
Bedell,  D.  C.  &  A,  C.  Testing,  Chap.  1. 


14 


DYNAMO  LABORATORY  OUTLINES 


THE  COMPOUND  GENERATOR 

The  object  of  this  experiment  is  to  study  the  compound 
dynamo  when  operated  as  a  generator. 

1.  Building  Up. — The  compound  generator  builds  up 
in  the  same  manner  as  the  shunt  machine. 

2.  Characteristics. — (a)  Connect    as   in   Fig.    5    and 
measure 


Load 


FIG.  5. 

(1)  terminal  voltage  for  outputs  up  to  150  per 
cent,  of  the  rated  load,  the  speed  and  the  field 
resistance  being  kept  constant. 

(2)  the  shunt  field  current. 

(b)  Determine 

(1)  the  resistance  of  the  shunt  field  circuit. 

(2)  the  resistance  of  the  armature  circuit. 

(3)  the  resistance  of  the  series  field  circuit. 

(c)  Plot  (1)  the  external  characteristic.  " 

(2)  the  internal  characteristic. 

(3)  the  RI  drop  in  the  armature. 

(4)  the  RI  drop  in  the  series  field. 

(d)  Define  and  calculate  the  regulation. 

3.  Calculation    of    Compounding. — The    number    of 
turns  in  the  series  field  winding  required  to   give   a 


4]  DIRECT  CURRENTS  15 

specified  degree  of  compounding  may  be  determined  by 
the  following  methods: 

(a)  from  the  armature  characteristic. 
(6)  by  added  turns. 

(a)  From  the  armature  characteristic. — Determine 
the  shunt  field  current  required  to  produce  the 
desired  voltage  at 

(1)  no-load. 

(2)  full-load. 
Then 


X-NiUi-1^* 

when  N  =  the  number  of  series  turns  required. 

Ni  =  the  number  of  turns  on  the  shunt  field. 

10  =  the  current  in  the  shunt  field  at  no-load. 

11  =  the  current  in  the  shunt  field  at  full-load. 
I  =the  current  in  the  series  field  at  full-load. 

Ei  =  ihe  full-load  voltage. 

jE/o^the  no-load  voltage. 

(6)  Added  Turns. — Over  the  shunt  field  wind  an 
auxiliary  field  of  any  convenient  number  of 
turns  of  insulated  wire,  connect  this  auxiliary 
field  winding  to  a  source  of  e.m.f.,  regulate  the 
shunt  field  to  give  the  rated  no-load  voltage  and 
determine  the  current  required  in  the  auxiliary 
winding  to  produce  the  desired  voltage  at  the 
rated  full-load  armature  current.  Then 


when  N  =  the  number  of  series  turns  required. 

Ari  =  the  number  of  turns  in  the  auxiliary  winding. 
Ii  =  the  current  in  the  auxiliary  winding. 
I  =the  current  in  the  series  winding  at  full-load. 


16  DYNAMO  LABORATORY  OUTLINES  [4 

4.  Determination  of  the  Number  of  Turns  in  the  Shunt 
Field  Winding.  —  (a)  Measure  the  field  current  and  the 
terminal  e.m.f.  at  some  given  speed. 

(b)  Over  the  field  coils  wind  an  auxiliary  field  of  a 
known  number  of  turns  and  determine  the 
current  required  in  this  winding  to  give  the 
same  e.m.f.  as  above  when  the  armature  is 
rotated  at  the  same  speed.  Then 


when  N  =  the  number  of  turns  in  the  shunt  field  winding. 
Ni  =  the  number  of  turns  in  the  auxiliary  winding. 
I  =the  current  in  the  shunt  field  winding. 
/i  =  the  current  in  the  auxiliary  winding. 

5.  Changing    the    Degree     of    Compounding.  —  The 

degree   of   compounding   of   a   given   dynamo   may   be 
changed  by 

(a)  shunting  the  series  field. 
(6)   changing  the  speed. 

(a)  Shunting  the  series  field.  —  Determine  the  degree 
of  compounding  when  the  generator  is  run  at 
rated  speed  and 

(1)  the  entire  load  current  flows  in  the  series  field 
windings. 

(2)  fifty  per  cent,  of  the  load  current  flows  in  the 
series  windings. 

(b)  Change    of   speed.  —  Determine    the    degree    of 
compounding  when  run  at 

(1)  rated  speed. 

(2)  twenty-five  per  cent,  above  rated  speed. 

(3)  twenty-five  per  cent,  below  rated  speed. 

(Note.  —  Regulate  the  shunt  field  current  so  that  the 
no-load  voltage  is  the  same  in  each  of  the  above  tests.) 


5]  DIRECT  CURRENTS  17 

6.  Explain 

(a)  the  difference  between  " short  shunt"  and  "long 
shunt"  compound  dynamos. 

(b)  why  the  ideal  compounding  is  not  obtained  in 
practice. 

(c)  the  effect  when  the  series  field  and  the  shunt 
field  are  opposed  and  give  an  example  of  the  use 
of  such  a  machine. 

(d)  the  use  of  the  "over"  compounded  generator. 

(e)  the  use  of  the  "flat"  compounded  generator. 


REFERENCES 

Franklin  &  Esty,  Direct  Currents,  Chap.  3. 
Smith,  Testing  of  Dynamos  &  Motors,  Chap.  8. 
Karapetoff,  Exp.  Elec.  Eng.,  Chap.  15. 
Bedell,  D.  C.  &  A.  C.  Testing,  Chap.  1. 


5 

CONDITIONS  AFFECTING  VOLTAGE 

The  object  of  this  experiment  is  to  determine  the 
effect  of  certain  factors  on  the  terminal  voltage  of  a 
generator. 

1.  Connect  a  shunt  dynamo  as  in  Fig.  2. 

2.  Brush  Position. — Drive  the  dynamo  at  constant 
speed  and  with  constant  field  excitation  but  with  the 
brushes  in  different  positions  and  note  the  voltmeter 
indications. 

3.  Magnetic  Leakage. — (a)  Note  the  voltage  when  the 
dynamo  is  driven  at  some  desired  speed  and  field  excita- 
tion. 

(6)  Connect  the  pole  pieces  by  means  of  iron  bars  or 
other  magnetic  material  and  note  the  voltage  at 
the  same  speed  and  field  excitation. 


18  DYNAMO  LABORATORY  OUTLINES  [5 

4.  Armature  Reaction  and  Armature  Drop. — (a)  Note 
the  no-load  voltage  for  a  given  speed  and  field  excitation 
(rated  values). 

(6)  Load  the  machine  and  determine  the  voltage  at 
the  same  speed  and  field  excitation. 

(c)  Determine  the  resistance  of  the  armature 
circuit  and  calculate  the  proportion  of  the 
total  voltage  drop  due  to 

(1)  armature  reaction. 

(2)  armature  resistance. 

5.  Speed. — With   constant   field  excitation,  note  the 
voltage  when  driven  at  different  speeds  up  to  125  per 
cent,  of  the  rated  speed. 

6.  Field  Excitation. — Drive  the  dynamo  at  its  rated 
speed  and  take  readings  of  the  voltage  for  field  currents 
varying  from  zero  up  to  150  per  cent,  of  that  required 
to  give  rated  voltage  at  no-load. 

7.  Plot  a  curve  using  voltage  as  ordinates  and  having, 
as  abscissa, 

(a)  speed. 

(6)  field  amperes. 

8.  Explain 

(a)  how   the   position   of   the   brushes  affects   the 

voltage. 
(6)   how  placing  an  iron  bar  across  the  pole  tips 

reduces  the  voltage. 

(c)  how  the  current  in  the  armature  affects  the 
voltage. 

(d)  how  the  voltage  varies  as  the  speed  changes. 

(e)  why  the  voltage  does  not  vary  directly  as  the 
field  excitation. 


6] 


DIRECT  CURRENTS 


19 


(/)  which  of  the  above  five  factors  combine  to 
produce  the  change  in  voltage  of  a  shunt 
generator  from  no-load  to  full-load. 

REFERENCES 

Franklin  &  Esty,  Direct  Currents,  pp.  82-85. 
Smith,  Testing  of  Dynamos  &  Motors,  Chap.  3. 

6 

PARALLEL   OPERATION  OF  GENERATORS 

The  object  of  this  experiment  is  to  study  the  action 
of  two  generators  when  supplying  a  common  load 
circuit. 

1.  Shunt  Generators. — (a)  Connect  two  shunt  gener- 
ators as  in  Fig.  6,  load  generator  A  to  its  rated  capacity 
and  regulate  the  voltage  to  the  rating  of  the  machine. 

To  Load 


FIG.  6. 

(6)  Start  generator  B  and  regulate  its  voltage 
until  it  is  equal  to  (or  slightly  greater  than) 
that  of  A.  Place  the  terminals  of  a  voltmeter 
across  the  open  switch  S.  If  the  voltmeter 
indication  is  zero,  the  switch  may  be  closed. 

(c)  After  closing  the  switch  S,  regulate  the  field  of 
B  until  the  machines  divide  the  load  in  pro- 
portion to  their  ratings,  increase  the  load  to  125 


20 


DYNAMO  LABORATORY  OUTLINES 


per  cent,  of  the  combined  ratings  of  the  two 
machines  and  read 

(1)  voltage. 

(2)  amperes  output  of  A. 

(3)  amperes  output  of  B. 

(d)  Repeat  the  readings  in  (c)  for  100  per  cent., 
75  per  cent.,  50  per  cent.,  25  per  cent.,  and  zero 
load,  without  changing  the  field  resistance  of 
either  machine. 

2.  Compound  Generators. — Connect  two  compound 
generators  as  in  Fig.  7  and  proceed  as  for  shunt 
generators. 

To  Load 


Equalizer 


FIG.  7. 


3.  Explain 


(a)  why,  after  closing  the  switch  S,  the  field  of 
the  shunt  generator  B  must  be  increased  to 
make  it  take  a  proper  portion  of  the  load. 

(6)  the  action  of  the  equalizer  when  compound 
generators  are  operated  in  parallel. 

(c)  the  advantages  of  parallel  operation. 

(d)  how  to  disconnect  a  machine  from  a  system 
when  operating  in  parallel  with  other  generators. 


7]  DIRECT  CURRENTS  21 

(e)  the  effect  when  the  field  excitation  of  one 
machine  is  reduced  so  that  its  voltage  is  less 
than  that  of  the  system. 

(/)  how  the  load  will  divide  between  two  machines, 
one  of  which  is  over-compounded  10  per  cent, 
and  the  other  25  per  cent.,  the  voltage  at  no- 
load  being  equal;  and  how  they  may  be  made 
to  divide  the  load  properly. 

(g)  the  effect  of  resistance  in  the  equalizer  circuit. 

(h)  why  the  equalizer  circuit  should  not  be  fused. 

(i)  the  effect  if  the  circuit  through  the  equalizer 
and  the  series  field  of  a  "dead"  machine  is 
not  broken. 

REFERENCES 

Franklin  &  Esty,  Direct  Currents,  pp.  184-198. 
Smith,  Testing  Dynamos  &  Motors,  pp.  118-121. 
Karapetoff,  Exp.  Elec.  Eng.,  Vol.  1,  pp.  263-269. 


THE  SHUNT  MOTOR 

The  object  of  this  experiment  is  to  study  the  shunt 
dynamo  when  operated  as  a  motor  and  to  obtain  data  for 
the  construction  of  the  performance  curves. 

1.  Direction  of  Rotation. — (a)  Connect  as  in  Fig.  8  and 
note  the  direction  of  rotation. 

(6)   Reverse  the  field  connections  and  note  the  direc- 
tion of  rotation. 

(c)  Reverse  the  armature  connections  and  note  the 
direction  of  rotation. 

(d)  Reverse  both  the  armature  and  the  field  connec- 
tions and  note  the  direction  of  rotation. 

2.  Performance   Curves  by  Loading. — (a)   Load  the 
motor  by  means  of  a  Prony  or  a  rope  brake  and  take 


22 


DYNAMO  LABORATORY  OUTLINES 


[7 


readings  of  the  following  quantities  for  loads  varying 
from  zero  to  150  per  cent,  of  the  rated  capacity,  the 
applied  voltage  being  kept  constant : 


Starting  Rheostat 


FlG. 


(1)  speed. 

(2)  armature  amperes. 

(3)  field  amperes. 

(4)  weight  on  scale. 

(6)  Calculate  and  tabulate 

(1)  torque. 

(2)  horse-power  output. 

(3)  efficiency. 

(4)  regulation. 

(c)   Using  horse-power  output  as  abscissa  plot  curves 
with  the  following  ordinates: 

(1)  efficiency. 

(2)  speed. 

(3)  torque. 

(4)  field  current. 

3.  Performance  Curves  from  the  Losses. — (a)  Supply 
the  motor  with  current  at  the  rated  voltage  and  measure 
(at  rated  speed) 

(1)  the  field  current  at  no-load. 

(2)  the  armature  current  at  no-load. 


7]  DIRECT  CURRENTS  23 

(6)   Determine  the  armature  resistance. 

(c)  Calculate  and  tabulate,  for  loads  up  to  150  per 
cent,  of  the  rated  capacity,  the  following: 

(1)  field  loss. 

(2)  armature  copper  loss. 

(3)  stray  power. 

(4)  torque. 

(5)  horse-power  output. 

(6)  speed. 

(7)  efficiency. 

(d)  Plot 

(1)  performance  curves  as  in  (2). 

(2)  loss  curves. 

4.  Explain 

(a)  why  the  speed  of  a  shunt  motor  falls  off  as  the 

load  increases. 
(6)  how  the  direction  of  rotation  of  a  shunt  motor 

may  be  changed. 

(c)  the  meaning  of  the  term  "  stray  power  "  and  how 
it  is  determined. 

(d)  the  counter  e.m.f.  of  a  motor  and  show  how  it 
is  automatically  adjusted  as  the  load  varies. 

(e)  the  relation  of  torque  and  armature  current. 
(0    the  effect  of  a  large  line  resistance  on  the  opera- 
tion of  the  motor. 

5.  Define 

(a)  speed  regulation. 

(6)  speed  control  and  name  the  methods  by  which 
the  speed  of  a  shunt  motor  may  be  controlled. 

REFERENCES 

Franklin  &  Esty,  Direct  Currents,  Chap.  4. 
Karapetoff,  Exp.  Elec.  Eng.,  Chap.  16. 
Smith,  Testing  of  Dynamos  &  Motors,  Chap.  8-9. 
Bedell,  D.  C.  &  A.  C.  Testing,  Chap.  2. 


24  DYNAMO  LABORATORY  OUTLINES  [8 

8 

THE  SERIES  MOTOR 

The  object  of  this  experiment  is  to  study  the  series 
dynamo  when  operated  as  a  motor  and  to  obtain  data 
for  the  construction  of  the  performance  curves. 

1.  Connect  as  in  Fig.  9,  supply  current  at  rated  voltage 
and  measure  the  following  quantities  for  loads  irom  150 
per  cent,  of  the  rated  capacity  to  the  highest  permissible 
speed: 

Starting  Rheostat 


FIG.  9. 

(a)  current. 

(6)  speed. 

(c)   weight  on  scale. 

(Warning. — Do  not  start  a  series  motor  without  load 
or  reduce  the  load,  while  running,  to  a  low  value,  as  the 
speed  will  become  excessive.) 

2.  Determine  the  resistance 

(a)  of  the  armature  circuit. 

(6)   of  the  field  circuit. 

(c)  between  adjacent  points  of  the  starting  rheostat. 

3.  Calculate  and  tabulate  for  various  loads 

(a)  torque. 

(6)  horse-power  output. 


8]  DIRECT  CURRENTS  25 

(c)  horse-power  input. 

(d)  efficiency. 

4.  Plot    curves,   using    torque    as    abscissa    and   the 
following  as  ordinates: 

(a)  armature  current. 
(6)  speed. 

(c)  efficiency. 

(d)  horse-power  output. 

5.  The  copper  losses  of  a  series  motor  may  be  calcu- 
lated for  any  given  value  of  armature  current  when  the 
combined  resistance  of  the  armature  and  the  field  circuit 
has  been  determined. 

The  stray  power  of  a  series  motor  varies  over  wide 
limits  since  both  the  speed  and  the  field  excitation  change 
as  the  load  changes.  It  may  be  determined,  for  any 
required  speed  and  field  excitation,  by  connecting  the 
armature  and  the  field  in  parallel  (converting  the  series 
into  a  shunt  motor)  and  to  a  supply  circuit  through  suit- 
able resistances  by  means  of  which  the  current  in  either 
winding  may  be  varied  independently  of  that  in  the 
other.  Excite  the  field  to  any  desired  degree  and  vary 
the  voltage  between  the  terminals  of  the  armature  until 
the  armature  rotates  at  the  required  speed.  The  input 
to  the  armature  is  the  sum  of  the  stray  power  and  the 
copper  loss  due  to  the  resistance  of  the  armature  winding. 

6.  Explain 

(a)  the  classes  of  service  to  which  the  series  motor 
is  adapted. 

(b)  the  law  by  which  the  torque  of  a  series  motor 
varies. 

(c)  why  the  upper  part  of  the  torque  curve  does  not 
follow  this  law. 


26 


DYNAMO  LABORATORY  OUTLINES 


[9 


(d)  the  variation  of  the  losses  in  a  series  motor  as 
the  load  changes. 

(e)  the  law  of  maximum  efficiency  (the  constant 
losses  equal  the  variable  losses)  for  the  series 
motor. 

REFERENCES 

Franklin  &  Esty,  Direct  Currents,  Chap.  4. 
Karapetoff,  Exp.  Elec.  Eng.,  Chap.  16. 
Smith,  Testing  of  Dynamos  &  Motors,  Chap.  10. 
Bedell,  D.-C.  &  A.-C.  Testing,  Chap.  2. 


9 

THE  COMPOUND  MOTOR 

The  object  of  this  experiment  is  to  determine  the  speed- 
torque  characteristics  of  the  compound  motor. 
1.  Connect  as  in  Fig.  10. 


Starting  Rheostat 


FIG.  10. 

With  the  switch  S  closed  in  the  upper  position,  load 
the  motor  by  means  of  a  Prony  or  a  rope  brake  and  deter- 
mine, for  armature  currents  up  to  150  per  cent,  of  the  full- 
load  rating,  the  following: 

(a)  speed. 
(6)  torque. 


10]  DIRECT  CURRENTS  27 

2.  Repeat   (1)   with  the  switch  closed   in  the  lower 
position. 

3.  Plot  curves  from  the  data  in  (1)  and  (2)  using  torque 
as  abscissa  and  speed  as  ordinates. 

4.  Explain 

(a)  why  the  cumulative  compound  motor  is  adapted 
to  the  driving  of  machine  tools  such  as  shears 
and  punches. 

(6)  to  what  class  of  service  the  differential  com- 
pound motor  is  adapted. 

(c)  how  the  differential  compound  motor  may  be 
made  to  give  a  good  starting  torque. 

(d)  the  relation  between  armature  current  and  tor- 
que in  (1)  the  cumulative  compound  motor,  (2) 
the  differential  compound  motor. 

5.  Compare  the  starting  torque  of  each  of  the  com- 
pound motors  with  that  of  the  shunt  and  of  the  series 
motor. 

REFERENCES 

Franklin  &  Esty,  Direct  Currents,  Chap.  4-5. 
Smith,  Testing  of  Dynamos  &  Motors,  Chap.  10. 
Karapetoff,  Exp.  Elec.  Eng.,  Chap.  16-17. 
Bedell,  D.-C.  &  A.-C.  Testing,  Chap.  2. 

10 

STATIC  TORQUE 

The  object  of  this  experiment  is  the  determination 
of  the  static  torque  of  a  motor. 

1.  Connect  a  shunt  or  a  series  motor  as  in  Fig.  11, 
clamp  a  brake  on  the  pulley  so  the  armature  cannot 
rotate  and  measure  the  pull  on  a  scale  for  ascending  and 
for  descending  values  of  armature  current  up  to  150  per 


28  DYNAMO  LABORATORY  OUTLINES  [11 

cent,  of  the  rated  capacity,  the  field  excitation  being 
kept  constant. 

2.  Repeat  (1)  for  different  field  excitations. 

3.  (a)  Calculate  the  torque  in  foot  pounds. 

(6)   Plot  curves  using  armature  current  as  ordinates 
and  torque  as  abscissa. 

4.  Explain 

(a)  the  meaning  of  the  term  " torque." 

(b)  what  factors  determine  the  torque  of  a  motor. 


FIG.  11. 

(c)  the  reason  for  the  difference  in  shape  of  the 
torque  curve  for  a  series  motor  and  that  for 
a  shunt  motor. 

(d)  why,   in   the   series   motor,    the   torque   curve 
approximates  a  straight  line  for  large  loads. 

(e)  why  the  static  torque  is  not  developed  at  the 
pulley  when  the  motor  is  running. 

(/)  why  the  torque  is  not  proportional  to  field 
excitation  (the  armature  current  remaining 
constant) . 

REFERENCES 

Franklin  &  Esty,  Direct  Currents,  pp.  98-99. 

Smith,  Testing  of  Dynamos  &  Motors,  pp.  123,  159-160. 


12]  DIRECT  CURRENTS  29 

11 

CONDITIONS  AFFECTING  THE  SPEED  OF  A  MOTOR 

The  object  of  this  experiment  is  to  determine  the 
effect  of  certain  conditions  on  the  speed  of  a  motor. 

1.  Connect  a  shunt  motor  as  in  Fig.  8  and  determine 
the  speed  of  its  armature. 

2.  Determine  the  speed 

(a)  for  a  forward  "lead"  of  the  brushes. 
(6)  for  a  backward  "lead"  of  the  brushes. 

(c)  when  the  pole  pieces  are  connected  by  means 
of  a  bar  of  iron  or  other  magnetic  material. 

(d)  when    the    applied    voltage    is    reduced    to 
approximately  one-half  that  in  (1). 

(e)  when  the  armature  voltage  is  as  in  (d)  and  the 
field  excitation  as  in  (1). 

(/)    when  the  field  resistance  is  increased. 
(g)    when  the  field  resistance  is  decreased. 

3.  Explain  each  change  of  speed  in  (2). 

State  which  of  the  above  methods  are  used  to  control 
the  speed  of  commercial  variable  speed  motors. 

REFERENCES    ' 

Franklin  &  Esty,  Direct  Currents,  pp.  103-112. 
Smith,  Testing  of  Dynamos  &  Motors,  Chap.  8. 
Karapetoff,  Exp.  Elec.  Eng.,  pp.  365-366. 

12 

STRAY  POWER 

The  object  of  this  experiment  is  the  determination 
of  the  stray  power  of  a  dynamo  and  the  separation  of 
such  loss  into  its  components. 


30 


DYNAMO  LABORATORY  OUTLINES 


[12 


1.  The  stray  power  of  a  dynamo  may  be  determined 
in  the  following  ways: 

(a)  by  means  of  an  auxiliary  motor. 

(b)  by  running  the  dynamo  as  a  motor. 

(c)  by  retardation. 

(a)  By  means  of  an  auxiliary  motor. — Connect 
as  in  Fig.  12  in  which  G  is  the  dynamo  the 
stray  power  of  which  is  to  be  determined  and 


'3. 


FIG.  12. 

M  is  an  auxiliary  motor  direct  connected  to 
the  shaft  of  G.  Keep  the  field  current  of  the 
motor  constant  throughout  the  test  by  means 
of  the  field  rheostat  and  regulate  the  speed  by 
means  of  the  resistance  in  the  armature  circuit. 

Drive  the  dynamo  at  its  rated  speed,  regulate  its 
field  (separate  excitation)  to  give  rated  voltage  and  de- 
termine the  watts  input  to  the  armature  of  the  motor,  M . 

Repeat  for  125  per  cent.,  75  per  cent.,  50  per  cent, 
and  25  per  cent,  of  rated  speed,  keeping  the  field  excitation 
constant. 

Determine  the  input  to  the  motor  armature  over  the 
above  range  of  speed  but  with  zero  field  excitation  on 
the  dynamo. 

Disconnect  the  motor  from  the  dynamo  and  determine 
the  losses  in  the  motor  armature  over  the  above  range 
of  speed. 


12]  DIRECT  CURRENTS 

Calculate  and  tabulate 

(1)  motor  armature  losses. 

(2)  the  friction  loss  in  the  dynamo. 

(3)  the  iron  (core)  loss  in  the  dynamo. 


31 


FIG.  13. 


Speed 

FIG.  14. 


Plot 


(1)  the  friction-loss  curve. 

(2)  the  iron-loss  curve. 

(b)  By  running  the  Dynamo  as  a  Motor. — Connect 
as  in  Fig.  13.  With  constant  field  excitation, 
run  at  various  speeds  from  125  per  cent,  of 


32  DYNAMO  LABORATORY  OUTLINES  [12 

rated  speed  to  as  low  a  value  as  possible  by 
changing  the  armature  resistance.  Read  arma- 
ture volts  and  amperes. 

Calculate  the  stray  power  and  plot  as  in  Fig.  14. 

A  tangent  to  the  stray-power  curve  drawn  through  the 
origin  will  separate  the  loss  into  its  components  of  iron 
and  friction  losses;  or  the  friction  may  be  determined  as 
follows : 


Field    Amperes 

FIG.  15. 

Keeping  the  speed  constant,  determine  the  stray  power 
for  different  field  excitations  and  plot  as  in  Fig.  15. 
Extend  the  curve  to  its  intersection  with  the  axis  of 
ordinates.  The  ordinate,  at  this  intersection,  is  the  loss 
due  to  friction  at  the  given  speed.  Repeat  for  not  less 
than  four  different  speeds  and,  from  the  friction  losses 
so  determined,  plot  the  friction-speed  curve  as  in  Fig.  14, 
which  will  divide  the  stray  power  into  iron  and  friction 
losses. 

(c)  Retardation. — Speed  up  a  shunt  motor  to 
approximately  125  per  cent,  of  the  rated  speed. 
Break  the  armature  circuit  and,  at  the  same 


12] 


DIRECT  CURRENTS 


33 


instant,  bring  the  field  rheostat  to  the  position 
that    will    give    the    desired    field    excitation. 
Determine  the  instantaneous  speed,  at  regular 
intervals,  as  the  armature  slows  down. 
Measure  the  watts  input  to  the  armature  at  this  exci 
tation  and  for  any  desired  or  convenient  speed ;  determine 
the  resistance  of  the   armature  winding  and  calculate 
the  stray  power. 


T  Time 

FIG.  16. 

Plot  the  speed-time  or  retardation  curve  as  in  Fig.  16. 

From  the  retardation  curve  find  speeds,  HI  and  n^, 

before  and  after  the  speed,  n,  for  which  the  wattage  was 

determined.     Select  n\  and  n2  so  that  — —~ — -  =  n. 
Then 


watts  loss  at  speed  n  =  K  1  ^ 

when  K  is  a  constant  proportional  to  the  moment  of 
inertia  of  the  rotating  parts. 


34  DYNAMO  LABORATORY  OUTLINES  [12 

and  T  is  the  time  required  for  the  rotating  parts  to 
decrease  from  speed  n\  to  w2. 

After  the  value  of  K  is  determined  from  the  above 
equation,  the  loss  at  any  required  speed  may  be  calcu- 
lated and  the  loss  curve  plotted  as  in  Fig.  17. 

Take  data  and  construct  the  retardation  curve  for 


Speed 

FIG.  17. 

(1)  normal  field  current. 

(2)  field  current  25  per  cent,  above  normal. 

(3)  field  current  25  per  cent,  below  normal. 

(4)  field  current  zero. 

From  the  above  curves  and  one  wattage  measurement 
determine  and  tabulate,  for  not  less  than  five  speeds,  the 
loss  for  each  degree  of  excitation. 

Plot 

(1)  watt-speed  curve  for  each  degree  of  excitation. 

(2)  watt-excitation  curve  showing  the  variation  of 
the  losses  with  field  excitation  but  for  constant 
(normal)  speed. 

(Note. — The  methods  outlined  above  for  determining 
the  stray  power  of  a  dynamo  are  not  limited  to  the  shunt 
machine  but  are  equally  applicable  to  the  series,  and  the 
compound  dynamo  and  to  alternating  current  machines.) 


12] 


DIRECT  CURRENTS 


35 


2.  Iron  Losses. — The  iron  (core)  losses  as  determined 
by  either  of  the  above  methods,  may  be  separated  into 
eddy-current  and  hysteresis  loss  in  the  following  manner : 
Iron  loss  =  hysteresis  loss + eddy-current  loss. 

Dividing  this  expression  by  n, 
W 


W 
which  is  an  equation  of  a  straight  line  between  —  and  n, 


Speed 

FIG.  18. 

when 

n  =  speed. 

kh=a  constant  porportional  to  the  hysteresis  loss. 

ke  =  a  constant  porportional  to  the   eddy-current 

loss. 
TF=the  total  iron  loss. 

From  the  iron  loss  curve  find  the  losses  for  a  series  of 
speeds  and  plot  as  in  Fig.  18.  Extend  the  line,  ab,  to 
the  axis  of  ordinates  and  through  the  intersection  draw 
a  horizontal  line.  The  ordinate,  oc,  multiplied  by  the 


36  DYNAMO  LABORATORY  OUTLINES  [13 

speed,  n,  gives   the  hysteresis  loss  at  that  speed;  the 
ordinate  between  ab  and  cd,  multiplied  by  the  speed  at 
which  the  ordinate  was  measured,  gives  the  eddy-current 
loss  at  that  speed. 
3.  Explain 

(a)  the  meaning  of  the  term  " stray  power"  and 
how  it  varies  (1)  in  a  motor,  (2)  in  a  generator. 
(6)  how  the  friction  loss  varies. 

(c)  how  the  iron  losses  vary. 

(d)  the  determination  of  the  " stray  power"  of  a 
(1)  series  motor,  (2)  compound  motor. 

REFERENCES 

Franklin  &  Esty,  Direct  Currents,  pp.  129-132. 

Karapetoff,  Exp.  Elec.  Eng.,  Chap.  17. 

Smith,  Testing  of  Dynamos  &  Motors,  pp.  210-227. 

Smith,  Alternating  Currents,  pp.  231-233. 

Bedell,  D.  C.  &  A.  C.  Testing,  Chap.  2. 

Foster's  Handbook. 

Standard  Handbook. 

13 

ARMATURE  MAGNETIZATION 

The    object    of    this    experiment    is    to    study    the 
magnetizing  action  of  the  armature  current. 

1.  Supply  the  armature  of    a    shunt    dynamo    with 
approximately  full-load  current,  the  field  circuit  being 
kept  open. 

Vary  the  position  of  the  brushes  through  180  elec- 
trical degrees  and  note  the  effect  on  the  armature. 

2.  Connect  the  field  terminals  of  the  dynamo  through 
a  voltmeter,  block  the  armature  to  prevent  rotation, 
supply  the  armature  with  current  as  in  (1)  and  note 
the    deflection    of   the    voltmeter   when   the    armature 
circuit  is  suddenly  broken. 


11]  DIRECT  CURRENTS  37 

Repeat  for  not  less  than  twenty  positions  of  the  brushes 
over  approximately  180  electrical  degrees. 

3.  Plot  a  curve  using  electrical  degrees  (or  commutator 
segments)    as    abscissa    and    voltmeter    deflections    as 
ordinates. 

4.  Explain 

(a)  the  action  of  the  armature  in  (1). 
(6)  the    deflection    of    the    voltmeter    when    the 
armature  circuit  is  broken  as  in  (2). 

(c)  why  there  is  no   deflection   of  the   voltmeter 
•    when  the  armature  circuit  is  broken  with  the 

brushes  in  the  " neutral"  position. 

(d)  why  the  voltmeter  deflection  is  not  the  same 
for  all  positions  of  the  brushes. 

(e)  why  the  terminals  of  the  voltmeter  must  be 
reversed  when  the  brushes  pass  the  "  neutral" 
position. 

(/)    the  meaning  of  the  term  "neutral"  position. 

REFERENCES 

Franklin  &  Esty,  Direct  Currents,  pp.  93,  151-161. 
Smith,  Testing  of  Dynamos  &  Motors,  pp.  62-64. 
Sheldon  &  Hausmann,  Direct  Currents,  Chap.  5. 
Karapetoff,  The  Magnetic  Circuit,  Chap.  9. 
Steinmetz,  Elements,  pp.  187-188. 

14 

MAGNETIZATION  CURVES 

The  object  of  this  experiment  is  to  obtain  data  for 
the  construction  of  the  magnetization  curve  at  no-load 
and  at  full-load. 

1.  At  No-load. — Connect  the  dynamo  to  be  tested 
as  in  Fig.  19,  the  armature  circuit  being  open  and  the 
field  separately  excited.  Drive  the  armature  at  con- 


38  DYNAMO  LABORATORY  OUTLINES  [14 

stant  (rated)  speed  and  take  readings  of  terminal  voltage 
for 

(a)  increasing  values  of  field  current. 
(6)   decreasing  values  of  field  current. 

2.  At  Full-load. — Connect   as   in    (1)    and   close   the 
armature  circuit  through  a  variable  resistance.     With 


FIG.  19. 

a  small  field  excitation,  adjust  the  resistance  of  the 
armature  circuit  until  the  rated  full-load  current  flows 
and  read  field  current  and  terminal  e.m.f. 

Increase  the  field  excitation,  simultaneously  increas- 
ing the  resistance  of  the  armature  circuit  so  that  the 
armature  current  remains  at  the  rated  full-load  value, 
and  again  read  field  amperes  and  terminal  volts. 

Repeat  for  small  increases  of  field  excitation  until  the 
field  approaches  saturation. 

3.  Construct 

(a)  the  no-load  magnetization  curve. 
(6)  the  full-load  magnetization  curve. 

4.  Explain 

(a)  why  the  magnetization  curves  for  ascending  and 
for  descending  values  of  field  current  do  not 
coincide. 

(6)  how  the  reading  may  be  corrected  for  any  vari- 
ation in  speed. 


15]  DIKECT  CURRENTS  39 

(c)  why  the  lower  part  of  the  magnetization  curve 
is  approximately  straight  while  the  upper  part 
bends  to  the  right. 

(d)  why  the  descending  no-load  curve  cuts  the  axis 
of  ordinates  above  the  origin  and  what  use  is 
made  of  this  fact  in  the  practical  operation  of 
dynamos. 

(e)  why  the  machine  should  be  separately  excited. 

REFERENCES 

Franklin  &  Esty,  Direct  Currents,  pp.  384-386. 
Smith,  Testing  of  Dynamos  &  Motors,  pp.  38-45. 
Karapetoff,'  Exp.  Elec.  Eng.,  Chap.  8. 
Bedell,  D.  C.  &  A.  C.  Testing,  Chap.  1. 
Steinmetz,  Elements,  pp.  188-190. 

15 

MAGNETIC  LEAKAGE 

The  object  of  this  experiment  is  to  determine  the  ratio 
of  the  total  flux  produced  by  the  field  windings  to  that 
effective  in  producing  electromotive  force  in  the  arma- 
ture conductors. 

1.  Connect  as  in  Fig.  20  for  a  bi-polar  dynamo.     A  is 
a  coil  of  insulated  wire  wound  over  the  field  winding  near 
the  yoke  and  B  is  a  coil,  having  the  same  number  of  turns 
as  A,  wound  on  the  armature  in  such  a  position  that  the 
flux  passing  through  the  armature  passes  through  the 
coil.     The  terminals  of  the  coils,  A  and  B,  should  be 
connected  to  a   low-reading  voltmeter  by  means  of  a 
double  pole,  double  throw  switch. 

For  multi-polar  dynamos  connect  as  in  Fig.  21,  making 
the  number  of  turns  in  coil  A  twice  the  number  in  coil  B. 

2.  Excite  the  field  weakly  from  a  suitable  source,  con- 
nect coil  A  to  the  voltmeter  and  note  the  throw  of  the 
pointer  when  the  armature  circuit  is  broken. 


40  DYNAMO  LABORATORY  OUTLINES  [15 


FIG.  20. 


Coil  A 


FIG.  21. 


16]  DIRECT  CURRENTS  41 

Connect  the  voltmeter  to  coil  B  and  determine  the 
throw  for  the  same  field  current. 

The  throw  of  the  voltmeter  pointer  is  proportional  to 
the  flux  threading  the  coil. 

3.  Repeat  (2)  for  values  of  field  current  up  to  25  per 
cent,  above  normal. 

4.  Calculate  the  leakage  coefficient  for  each  value  of 
field  current. 

Plot  a  curve  using  field  current  as  abscissa  and  leakage 
coefficient  as  ordinates. 

5.  Explain 

(a)  why  the  leakage  coefficient  is  not  constant. 

(6)  why  the  voltmeter  indicates  only  when  the  field 

circuit  is  being  opened  or  closed, 
(c)   why  one  coil  should  have  twice  as  many  turns 

as  the  other  when  a  multi-polar  dynamo  is  to 

be  tested. 

REFERENCES 

Franklin  &  Esty,  Direct  Currents,  pp.  370-376. 

Smith,  Testing  of  Dynamos  &  Motors,  pp.  51-52,  258-260. 

Karapetoff,  Exp.  Elec.  Eng.,  pp.  181-182. 

Sheldon  &  Hausmann,  Direct  Currents,  pp.  94-96. 

16 

HYSTERESIS  LOOP  AND  LENGTH  OF  AIR  GAP 

The  object  of  this  experiment  is  the  construction  of 
the  hysteresis  loop  and  the  determination  of  the  length 
of  the  air  gap. 

1.  Hysteresis  Loop. — (a)  Connect  as  in  Fig.  19  and 
read  field-current  and  terminal  e.m.f.  for  a  complete 
magnetic  cycle. 

(6)   Construct  the  hysteresis  loop  as  in  Fig.  22. 


42 


DYNAMO  LABORATORY  OUTLINES 


[16 


2.  Length  of  the  Air  Gap. — Through  the  origin  and 
parallel  to  the  straight  portion  of  the  hysteresis  loop, 
draw  the  line  ab.  The  ratio  of  the  abscissa  of  any  point 
on  this  line  and  the  ordinate  of  the  same  point  is  the 


FIG.  22. 


/  . 


ratio  of,  and  may  be  substituted  for  ™  in  the  following 
expression : 


_ 


_ 

E 


when   I  =the  length  of  the  air  gap  in  centimeters. 
N  =  the  number  of  turns  per  pair  of  field  poles. 
Z  =the  total  number  of  armature  conductors. 
n  =  armature  revolutions  per  minute. 


17]  DIRECT  CURRENTS  43 

A  =  the  sectional  area  of  the  air  gap  in  square  cen- 
timeters. 

pi  =  the  number  of  field  poles. 
p2  =  the  number  of  parallel  paths  through  which  the 

current  may  flow. 
7   =the  field  current  (amperes). 
E  =  voltage  generated  in  the  armature. 

3.  (a)  Determine  the  length  of  the  air  gap. 

(6)  Derive  the  formula  for  the  length  of  the  air  gap. 

4.  Explain 

(a)  why  the  hysteresis  curve  is  a  loop. 
(6)  why  the  sides  of  the  loop  bend  over  as  the 
excitation  increases. 

(c)  how  you  would  judge  the  quality  of  iron  from 
its  hysteresis  loop. 

(d)  the  effect  of  the  length  of  the  air  gap  on  the  shape 
of  the  hysteresis  loop. 

(e)  the  effect  of  the  length  of  the  air  gap  on  the 
magnetizing    force    (ampere    turns)     required 
for  a  given  voltage  in  a  given  machine. 

REFERENCES 

Franklin  &  Esty,  Direct  Currents,  pp.  279-281. 

Karapetoff,  Exp.  Elec.  Eng.,  Chap.  8-9. 

Sheldon  &  Hausmann,  Direct  Currents,  pp.  38-40. 

17 

FLUX  DISTRIBUTION  IN  THE  AIR  GAP 

The  object  of  this  experiment  is  to  determine  the 
distribution  of  the  flux  in  the  air  gap  when  the  armature 
is  unloaded  and  when  it  is  loaded. 

1.  Single  Pilot  Brush. — (a)  Support  an  auxiliary 
brush,  B,  (Fig.  23)  so  that  it  will  make  contact  with  the 


44  DYNAMO  LABORATORY  OUTLINES  [17 

commutator  and  that  its  position,  relative  to  the  main 
brushes,  A  A,  may  be  varied  by  regular  steps. 

(6)  Drive  the  dynamo  at  its  rated  speed  and  with 
such  field  excitation  as  will  give  rated  voltage. 
Vary  the  position  of  the  auxiliary  brush,  by 
regular  steps,  through  approximately  180  elec- 
trical degrees,  and  take  a  voltmeter  reading  for 
each  position.  The  voltmeter  reading  is  the 
sum  of  the  voltages  generated  in  the  armature 


FIG.  23. 

coils  between  the  auxiliary  brush  and  the  main 
brush  to  which  the  voltmeter  is  connected. 
(c)    Repeat  (b)  when  the  armature  is  carrying  full- 
load  (rated)  current. 

2.  Double  Pilot  Brush. — Replace  the  single  brush  in 
(1)  with  two  brushes,  insulated  from  each  other  and  so 
spaced  that  the  distance  from  center  to  center  of  the 
brushes  is  equal  to  that  from  center  to  center  of  two 
adjacent  commutator  bars.  (Fig.  24.)  Vary  the  posi- 
tion of  the  pilot  brushes  as  in  (1)  and  note  the  voltmeter 
indications. 


17]  DIRECT  CURRENTS  45 

3.  With  e.m.f.  as  ordinates  and  electrical  degrees 
(or  commutator  segments)  as  abscissa,  construct  a 
diagram  showing  the  distribution  of  the  flux  for 

(a)  no-load. 
(6)  full-load. 

indicating  on  the  diagram  the  position  of  the  pole  and  of 
the  main  brushes. 


FIG.  24. 

4.  Explain 

(a)  how  to  determine  the  proper  position  of  the 

brushes. 
(6)  why  the  relative  position  of  the  pole  and  the 

brush  is  not  the  same  at  full-load  as  at  no-load. 

(c)  why  the  distribution  of  the  flux  in  the  air  gap  is 
not  the  same  at  full-load  as  at  no-load. 

(d)  how  the  voltage  measured  is  an  indication  of  the 
magnitude  of  the  flux. 

REFERENCES 

Karapetoff,  Exp.  Elec.  Eng.,  pp.  182-184. 
Steinmetz,  Elements,  pp.  177-186. 


46  DYNAMO  LABORATORY  OUTLINES  [18 

18 

BOOSTER  ACTION 

The  object  of  this  experiment  is  to  study  the  booster 
dynamo  (1)  when  used  to  compensate  for  line  drop,  (2) 
when  used  in  connection  with  a  storage  battery  to 
equalize  the  load  on  a  generator. 

1.  Line  Booster. — Connect  as  in  Fig.  25,  the  booster 
being  a  series  generator  (whose  armature  conductors 
are  sufficiently  large  to  carry  the  entire  line  current) 
driven  by  a  shunt  motor  or  other  constant  speed  engine. 


Booster 


Load 


FIG.  25. 

The  resistance,  R\,  is  a  rheostat,  the  drop  through  which 
represents  that  in  a  long  feeder  for  which  the  booster 
is  to  compensate. 

(a)  Load  the  system  by  means  of  a  resistance,  regu- 
late the  shunt  around  the  field  of  the  booster 
so  that  the  voltage  at  the  load  terminals  is  equal 
to  that  at  the  terminals  of  the  generator.  Vary 
the  load  on  the  generator  from  zero  to  125  per 
cent,  of  the  rated  capacity  reading,  for  each  step, 

(1)  e.m.f.  at  generator  terminals. 

(2)  e.m.f.  at  load  terminals. 

(3)  line  amperes. 

(6)  Using  line  current  as  abscissa,  plot  a  curve 
having  as  ordinates 


18] 


DIRECT  CURRENTS 


47 


(1)  generator  voltage. 

(2)  load  voltage. 

2.  Battery  Booster. — Connect  as  in  Fig.  26,  the 
booster  being  a  differentially  compounded  generator. 
Regulate  the  shunt  field  of  the  booster  so  that  the 
booster  voltage  will  be  approximately  one-fifth  that  of 
the  battery.  Regulate  the  shunt  around  the  series 
(booster)  field  so  that  this  field  will  neutralize  the  shunt 
field  at  the  average  load  to  be  delivered  to  the  receiving 
circuit. 


Load 


FIG.  26. 

Vary  the  load,  by  small  steps,  from  zero  to  the  max- 
imum, and  read 

(a)  line  amperes. 

(6)  battery  amperes  (to  or  from). 

(c)   generator  amperes. 

Repeat,  varying  the  load  as  rapidly  as  possible  or  by 
sudden  increments. 

Plot  curves  showing  the  relation  between  (a),  (b) 
and  (c). 

3.  Explain 

(a)  the  action  of  the  line  booster. 

(b)  the  action  of  the  battery  booster. 


48  DYNAMO  LABORATORY  OUTLINES  [19 

REFERENCES 

Franklin  &  Esty,  Direct  Currents,  pp.  256-265. 
Smith,  Testing  of  Dynamos  &  Motors,  pp.  296-306. 
Karapetoff,  Exp.  Elec.  Eng.,  Vol.  2,  pp.  414-420. 
Sheldon  &  Mason,  D.-C.  Machinery,  Chap.  8. 
Lyndon,  Storage  Batteries,  Chaps.  31-2-3. 
Crocker,  Electric  Lighting,  pp.  424-428. 

19 

HEAT  TEST 

The  object  of  this  experiment  is  to  determine  the 
rise  in  temperature  of  the  different  parts  of  a  dynamo 
under  load  conditions. 

1.  Determine  the  resistance  of  the 

(a)  armature  winding. 
(6)  field  winding. 

2.  Load  the  dynamo  as  either  a  generator  or  a  motor 
by  means  of  a  brake  (motor),  a  water  rheostat  (genera- 
tor) or  by  any  of  the  opposition  methods  (either  motor 
or  generator). 

Maintaining  the  speed,  the  voltage  and  the  arma- 
ture current  at  the  rated  values,  make  periodical  de- 
terminations of  the  following: 

(a)  temperature  of  the  air. 

(6)  temperature  of  the  field  windings. 

(c)  temperature  of  the  leading  pole  tip. 

(d)  temperature  of  the  lagging  pole  tip.  - 

(e)  temperature  of  the  bearings. 

(/)    temperature  of  the  armature  core. 
(g)  resistance  of  the  field  windings. 
(h)  resistance  of  the  armature  winding. 

3.  Using  a  temperature  coefficient  of  0.0042  calculate 
the  temperature  rise  of 


20]  DIRECT  CURRENTS  49 

(a)  the  field  windings. 

(b)  the  armature  winding. 

With  time  as  abscissa  and  temperature  as  ordinates, 
plot  curves  showing  the  temperature  rise  of  each  of  the 
above  parts. 

Correct  the  observed  temperature  rise  to  the  standard 
temperature  of  25°  C.  (i.e.,  calculate  the  rise  that  would 
occur  if  the  temperature  of  the  air  were  25°  C.)  by  adding 
or  subtracting  one-half  of  1  per  cent,  for  each  degree  that 
the  air  is  below  or  above  the  standard  temperature. 

4.  Explain 

(a)  why  the  trailing  and  the  leading  pole  tips  do  not 

have  the  same  temperature. 
(6)   why  the  rise  in  the  temperature  of  the  coils,  as 

observed,  is  different  from  that  calculated  from 

the  change  in  resistance. 

(c)  how  the  copper  loss  changes  with  increased  tem- 
perature. 

(d)  how  the  iron  losses  change  with  increased  tem- 
perature. 

REFERENCES 

Franklin  &  Esty,  Direct  Currents,  pp.  148-150. 
Karapetoff,  Exp.  Elec.  Eng.,  Chap.  18. 


20 

THREE-WIRE  SYSTEM 

The  object  of  this  experiment  is  to  determine  the  volt- 
age relations  in  a  three-wire  system. 

1.  (a)  Connect  a  rotary  converter  as  in  Fig.  27  or 
two  similar  shunt  dynamos  as  in  Fig.  28. 

(6)  Load  the  three-wire  system  so  that  the  indica- 


50 


DYNAMO  LABORATOKY  OUTLINES 


[20 


tions  of  the  ammeters  in  the  outside  wires  are 
equal. 

(c)  Decrease  the  load  on  one  side  of  the  system  to 
zero,  recording  the  ammeter  and  the  voltmeter 
indications  for  not  less  than  five  steps. 


FIG.  27 

(d)  Reduce  the  load  on  the  other  side  of  the  system 

in  a  similar  manner. 
2.  Using  total  load  on  the  system  as  abscissa,  plot 

curves  showing  the  voltage  between  each  outside  wire 
and  the  neutral. 


FIG.  28. 

3.  Explain 

(a)  why  the  current  in  the  neutral  wire  is  zero  when 

the  currents  in  the  outside  wires  are  equal. 
(6)  the  advantages  of  the  three- wire  system. 


21]  DIRECT  CURRENTS  51 

(c)  the  effect  of  the  inductance  coil  in  the  rotary 
converter  connection. 

(d)  why  the  regulation  of  the  three-wire  generator 
is  better  than  that  of  two  shunt  dynamos  in 
series. 

(e)  how   the    three-wire   generator   may    be    com- 
pounded. 

REFERENCES 

Franklin  &  Esty,  Direct  Currents,  pp.  269-275. 
Smith,  Testing  of  Dynamos  &  Motors,  pp.  284-292. 
Karapetoff,  Exp.  Elec.  Eng.,  Vol.  1,  pp.  291-293. 

21 

THE  MOTOR-GENERATOR  AND  THE  DYNAMOTOR 

The  object  of  this  experiment  is  to  determine  the  regu- 
lation and  the  efficiency  of  a  motor-generator  or  of  a 
dynamotor. 

1.  Keeping  the  voltage  at  the  terminals  of  the  motor 
of  a  motor-generator  constant,  run  the  set  at  rated  speed 
and  determine,  for  loads  up  to  125  per  cent,  of  rated 
capacity: 

(a)  input. 

(6)  output. 

(c)   the  voltage  at  the  generator  terminals. 

2.  Repeat  with  constant  field  excitation  of  the  motor, 

determining,  in  addition  to  (a),  (6)  and  (c)  of 
(1),  the  speed  of  the  motor. 

3.  Repeat  using  a  dynamotor  instead  of  a  motor-gen- 
erator. 

4.  With  current  output  as  abscissa  plot  curves  with 
the  following  ordinates: 


52  DYNAMO  LABORATORY  OUTLINES  [22 

(a)  efficiency. 

(6)  voltage  of  generator. 

(c)   speed  of  motor. 

5.  Calculate  the  per  cent,  regulation  of  the 

(a)  motor. 

(6)  generator  with  constant  speed. 
(c)   generator  with  constant  field  excitation  of  the 
motor. 

6.  Explain 

(a)  why  the  efficiency  is  greater  in  a  motor-gener- 
ator, at  constant  speed  than  at  constant  excita- 
tion of  the  motor  field. 

(6)  why  the  voltage  regulation  of  the  generator  is 
better  at  constant  speed  than  at  constant  excita- 
tion of  the  motor  field. 

(c)  why  the  efficiency  of  the  dynamotor  is  greater 
than  that  of  the  motor-generator. 

(d)  the  structural  differences  in  the  motor-generator 
and  the  dynamotor. 

(e)  the    comparative    advantages    of    the    motor- 
generator  and  the  dynamotor. 

(/)  by  means  of  a  diagram,  the  connections  for 
using  the  dynamotor  as  a  three-wire  generator. 

REFERENCES 

Franklin  &  Esty,  Direct  Currents,  70-71. 

Smith,  Testing  of  Dynamos  &  Motors,  pp.  292-296. 

Sheldon  &  Mason,  D.-C.  Machinery,  Chap.  8. 

22 

INSULATION  RESISTANCE 

The  object  of  this  experiment  is  the  determination  of 
the  resistance  of  insulation. 


22] 


DIRECT  CURRENTS 


53 


1.  Connect  as  in  Fig.  29.  When  the  single-pole, 
double-throw  switch  is  closed  in  the  upper  position,  the 
voltmeter  is  connected  directly  across  the  supply  lines; 
when  closed  in  the  lower  position,  the  insulation  to  be 
tested  is  connected  across  the  supply  lines  in  series  with 
the  voltmeter. 

(Note. — The  voltage  used  in  this  test  should  not  be 
less  than  the  normal  working  pressure  of  the  apparatus 


—  -  Insulation 


FIG.  29. 


tested  and  the  voltmeter  should  have  a  high  resistance. 
A  Weston  voltmeter  having  a  resistance  of  from  60,000 
to  80,000  ohms  is  suitable.) 


R  = 


when  R  =the  resistance  of  the  insulation  under  test. 
Rv  =  the  resistance  of  the  voltmeter. 
EI  —  voltmeter  indication  when  connection  is  made 

directly  across  the  supply  lines. 
E2  =  the  voltmeter  indication  when  the  voltmeter 

and  the  insulation  are  connected  in  series. 
2.  Determine  the  resistance  of  the  insulation  between 
(a)  the  armature  winding  of  the  assigned  dynamo 

and  the  frame. 

(6)  the  field  winding  of  the  assigned  dynamo  and 
the  frame. 

(c)  the  wires  of  the  supply  circuit. 

(d)  the  wire  of  the  supply  circuit  and  the  ground. 


54  DYNAMO  LABORATORY  OUTLINES  [23 

3.  Explain  why  a  high-resistance  voltmeter  is  necessary 
in  this  experiment. 

REFERENCES 

Smith,  Testing  of  Dynamos  &  Motors,  pp.  255-257. 
Karapetoff,  Exp.  Elec.  Eng.,  Vol.  2,  pp.  82-88. 
Standard  Handbook. 
Foster's  Handbook. 

23 

THE  VARIABLE  SPEED  MOTOR 

The  object  of  this  experiment  is  to  study  the  operating 
characteristics  of  a  variable  speed  shunt  motor. 

1.  Load  a  variable  speed  shunt  motor  to  its    rated 
capacity  at  its  lowest  speed  and  determine 

(a)  input. 

(6)   output. 

(c)   speed. 

Reduce  the  load  50  per  cent,  and  take  a  second  set  of 
readings. 

Measure  the  speed  at  no-load. 

2.  Repeat  (1)  for  different  speeds  up  to  the  maximum 
allowable. 

3.  Determine  for  each  speed 
(a)  the  efficiency  at  full-load. 
(6)  the  regulation. 

4.  Explain 

(a)  by  means  of  a  diagram,  the  construction  and  the 
operation  of  the  motor  tested. 

(b)  the  general  principles  of  other  variable  speed 
(commercial)  motors. 

REFERENCES 

Franklin  &  Esty,  Direct  Currents,  pp.  105-112. 
Smith,  Testing  of  Dynamos  &  Motors,  pp.  128-130. 
Karapetoff,  Exp.  Eng.,  Vol.  1,  pp.  291-293. 


ALTERNATING  CURRENTS 


PROPERTIES  OF  A  CIRCUIT  CARRYING  AN 
ALTERNATING  CURRENT 

The  object  of  this  experiment  is  to  show  the  effect 
of  resistance  and  of  reactance  in  a  circuit  carrying  an 
alternating  current. 

Part  1. — Resistance  and  Reactance  in  Series. 

1.  (a)  Connect  an  inductive  and  a  non-inductive 
resistance  in  series,  as  shown  in  Fig.  30,  and  determine 


FIG.  30. 

the  following  when  the  circuit  is  supplied  with  current 
at  60  cycles  and  a  suitable  voltage: 

(1)  amperes. 

(2)  watts. 

(3)  voltage  ac. 

(4)  voltage  ab. 

(5)  voltage  be. 

(b)   Change  the  value  of  the  inductance  and  take 
a  second  set  of  readings. 
55 


56  DYNAMO  LABORATORY  OUTLINES  [1 

(c)   Change  the  value  of  the  resistance  and  take  a 
third  set  of  readings. 

(Note. — The  inductive  resistance  should,  preferably, 
be  a  coil  without  iron  and  so  constructed  that  its  in- 
ductance may  be  changed  without  changing  the  ohmic 
resistance  of  the  circuit.) 

2.  Replace  the  inductive  resistance  with  a  coil 
having  a  removable  iron  core  and  take  two  sets  of 


readings — one  with  the  iron  core  in  the  coil  and  one  with 
it  removed — the  current  or  the  applied  voltage  being 
the  same  in  both  cases. 

3.  Determine   the    ohmic    resistance   of   the    circuits 
in  (1)  and  in  (2). 

4.  Replace  the  inductance   coil  in   (1)   with  a  con- 
denser and  take  readings  for  two   different   values  of 
capacity. 

5.  Change  the  frequency  and  repeat  (1),  (2)  and  (4). 

6.  Construct    the    voltage    triangle    from    the    data 
obtained  in  (1),   (2),   (4)  and  (5)  as  in  Fig.  31.     The 
projection,  ad,  of  the  line,  ac,  on  the  line,  ab,   should 

equal  approximately,  jy. 

7.  Discuss    the    changes    in    the    meter    indications 
when 

(a)  the  inductance  is  changed. 


1]  ALTERNATING  CURRENTS  57 

(6)  the  capacity  is  changed. 

(c)  the  frequency  is  changed. 

(d)  the  iron  core  is  removed  from  the  coil. 

8.  Calculate 

(a)  the  value  of  the  non-inductive  resistance. 
(6)  the  ohmic  resistance  of  the  inductive  coil. 
(c)  the  inductance  (or  capacity)  of  the  circuit. 

9.  Explain 

(a)  the  meaning  of  the  lines  bd  and  cd  in  the 

voltage  diagram. 
(6)   how  the  presence  of  iron  affects  the  inductance 

of  the  circuit. 
(c)   why  the  ohmic  resistance  of  the  circuit  in  (2) 

W 
is  less  than  -™. 

10.  Compare 

(a)  the  terminal  voltage  and  the  power  lost  when  an 
alternating  current  flows  in  an  inductive  circuit 
without  iron,  with  the  terminal  voltage  and  the 
power  lost  when  a  direct  current  of  the  same 
value  flows  in  the  same  circuit. 

(6)  the  terminal  voltage  and  the  power  lost  when  an 
alternating  current  flows  in  a  coil  without  iron, 
with  the  terminal  voltage  and  the  power  lost 
when  the  same  value  of  alternating  current 
flows  in  the  same  coil  but  with  an  iron  core. 

Part  2. — Resistance  and  Reactance  in  Parallel. 

1.  (a)  Connect  an  inductive  and  a  non-inductive 
resistance  in  parallel,  as  shown  in  Fig.  32,  and  note  the 
indications  of  the  three  ammeters  when  connection  is 
made  to  a  60-cycle  alternating-current  system  of  the 
proper  voltage. 


58 


DYNAMO  LABORATORY  OUTLINES 


[1 


(6)  Change  the  value  of  the  inductance  and  take 
a  second  set  of  readings. 

(c)  Change  the  value  of  the  resistance  and  take  a 
third  set  of  readings. 


FIG.  32. 


2.  Replace  the  inductance  with  a  capacity  and  repeat 


(1). 


3.  Repeat  (1)  and  (2)  for  a  different  frequency. 

4.  Construct  the  current  triangle. 

5.  Determine 

(a)  the  resistance  of  the  circuit. 
(6)  the  impedence  of  the  circuit. 

(c)  the  inductance  (or  capacity)  of  the  circuit. 

(d)  the  power  factor  of  the  circuit  and  that  of  the 
inductance  coil. 

6.  Explain 

(a)  why  the  current  in  the  line  is  not  the  sum  of  the 
currents  in  the  branches. 

(6)  what  practical  condition  the  above  circuit  repre- 
sents. 

(c)   the  following  equation: 


1]  ALTERNATING  CURRENTS  59 

when  Ri  and  R2  are  the  resistances  of  two  coils  connected 
in  parallel. 

Xi  and  X2  are  the  reactances   of  the  two  coils. 
Zi  and  Z2  are  the  impedences  of  the  two  coils. 
Z  is  the  impedence  of  a  single  coil  which  is  equiva- 
lent to  the  two  coils  in  parallel. 
(d)  how  the  resistance   and  the  reactance  of  the 
equivalent  coil  may.  be  determined. 

Part  3. — Voltage  Resonance. 

1.  Connect  a  non-inductive  resistance,  an  inductive 
resistance  and  a  condenser  as  in  Fig.  33,  supply  an  al- 
ternating current  and  read  the  following  for  different 
values  of  inductance : 


c 
C  = 


FIG.  33. 

(a)  amperes. 
(6)  voltage  ab. 

(c)  voltage  be. 

(d)  voltage  cd. 

(e)  voltage  ac. 
(/)    voltage  ad. 

2.  Repeat  (1)  varying  the  capacity. 

3.  Repeat  (1)  varying  the  frequency. 

4.  Construct  the  voltage  triangle,  abc,  (Fig.  34)  and 


60  DYNAMO  LABORATORY  OUTLINES  [1 

from  c  drop  a  perpendicular,  cd,  proportional  to  Ecd. 
The  line,  ad,  should  be  proportional  to  Ead,  the  applied 
voltage. 

5.  Calculate  the  value  of 

(a)  the  ohmic  resistance  of  each  part  of  the  circuit. 

(b)  the  inductance  of  the  circuit. 

(c)  the  capacity  of  the  circuit. 

(d)  the  reactance  of  the  circuit. 

(e)  the  power  factor  of  the  circuit  and  of  each  part. 


b 
FIG.  34. 

6.  Construct  a  curve  with  inductance,  capacity  or  fre- 
quency as  abscissa  and 

(a)  current  as  ordinates. 

(b)  power  factor  as  ordinates. 

7.  Explain 

(a)  the  effect  of  varying  the  frequency. 

(b)  the  effect  of  varying  the  inductance. 

(c)  the  effect  of  varying  the  capacity. 

(d)  the  advantages  and  the  disadvantages  of  voltage 
resonance. 

(e)  how  the  voltage  over  a  part  of  the  circuit  may 
be  greater  than  that  over  the  whole. 

Part  4. — Current  Resonance. 

1.  Connect  a  non-inductive  resistance,  an  inductive 
resistance  and  a  capacity  to  A.-C.  mains  as  indicated  in 
Fig.  35.   Vary  the  inductance  and  read  volts  and  amperes. 

2.  Repeat  (1)  varying  the  capacity. 


1] 


ALTERNATING  CURRENTS 


61 


3.  Repeat  (1)  varying  the  frequency. 

4.  Determine  the  power  factor  of  the  circuit  for  each 
value  of  inductance,  capacity  or  frequency. 

5.  Plot  a  curve  using  inductance,  capacity  or  frequency 
as  abscissa  and 

(a)  line  current  as  ordinates. 
(6)  power  factor  as  ordinates. 


-K5HVWWW1 


0     0 


FIG.  35. 

6.  Explain 

(a)  the  advantages  of  current  resonance. 

(6)  the  disadvantages  of  current  resonance. 

(c)   how  the  line  current  may  be  less  than  the  sum 

of  the  currents  in  the  branches. 
'(d)  how   current   resonance    may   be    obtained    in 

transmission  lines. 

REFERENCES 

Franklin  &  Esty,  Alternating  Currents,  Chap.  4. 
Karapetoff,  Exp.  Elec.  Eng.,  Chap.  5. 
Karapetoff,  The  Electric  Circuit,  Chaps.  4-7. 
Steinmetz,  A.-C.  Phenomena,  Chap.  9. 
Steinmetz,  Elements,  pp.  48-58. 
Smith,  Alternating  Currents,  Chap.  1-5. 
Foster's  Handbook. 
Standard  Handbook. 


62 


DYNAMO  LABORATORY  OUTLINES 


[2 


THE  ALTERNATING-CURRENT  GENERATOR 

The  object  of  this  experiment  is  to  obtain  data  for  the 
construction  of  the  saturation  curve  and  of  the  short- 
circuit  curve,  and  for  the  calculation  of  the  efficiency 
and  the  regulation  of  an  alternating-current  generator. 

1.  The  Saturation  Curve. — Connect  an  alternator  as 
in  Fig.  36  (switches  open)  and  take  readings  of  the 


Single  Phase 


Three  Phase 
FIG.  36. 

e.m.f.  of  the  armature  (running  at  rated  speed)  for  field 
currents  varying  from  zero  to  150  per  cent,  of  normal 
excitation  or  until  the  field  approaches  saturation. 

2.  The  Short-circuit  Curve. — Connect  an  alternator 
as  in  Fig.  36  (switches  closed),  the  armature  being  short- 
circuited  through  a  suitable  ammeter.  Vary  the  field 
excitation  (beginning  at  zero)  so  as  to  obtain  values  of 
armature  current  up  to  150  per  cent,  of  full-load. 


2] 


ALTERNATING  CURRENTS 


63 


3.  Determine  the  resistance  of  the  armature. 

4.  Synchronous  Reactance. — Divide  the  open-circuit 
voltage   (as  taken  from  the  saturation  curve)   by  the 
short-circuit  amperes  produced  by  the  same  value  of 
field  excitation.     This  gives  the  synchronous  impedence 
from  which  the  synchronous  reactance  may  be  determined, 
using  the  armature  resistance  as  determined  in  (3). 


Saturation  (Magnetization)  Curve 


Short-Circuit  Curve 


Field  Amperes 

FIG.  37. 

5.  Regulation. — The  regulation  of  an  alternator  may 
be  determined  by  (a)  loading,  (b)  the  e.m.f.  method,  (c) 
the  m.m.f.  method. 

(a)  Loading. — Load  the  alternator  (the  load  circuit 
having  some  specified  power  factor)  to  its  rated 
capacity,  the  speed  and  voltage  of  the  machine 
being  at  the  rated  values. 

Determine  the. voltage  when  the  load  is  reduced  to 
zero,  the  speed  and  the  field  excitation  remaining  constant. 
(See  A.I.E.E.  Rules.) 

(6)  E.M.F.  Method. — From  the  short-circuit  curve 
and  the  saturation  curve  find  the  voltage  re- 


64  DYNAMO  LABORATORY  OUTLINES  [2 

quired  to  force  full-load  current  through  the 
short-circuited  armature.  The  no-load  voltage 
is  the  vector  sum  of  this  e.m.f.  and  the  rated 
voltage  of  the  alternator,  taking  account  of  the 
power  factor  of  the  load  circuit. 

E  = 


when  E  =the  no-load  voltage. 

EI  =  the  rated  voltage  of  the  alternator. 
RI  =  resistance  drop  in  the  alternator  armature. 
XI  =  reactance  drop  in  the  alternator  armature. 
cos  (/)  =  power  factor  of  the  load  circuit. 


150% 


FIG.  38. 

(c)  M.M.F.  Method. — Find,  from  the  saturation 
curve,  the  field  current  required  to  produce  a 
voltage  equal  to  the  total  ohmic  drop  in  the 
circuit  (Ei  cos  <f>-\-RI)  and,  from  the  saturation 
curve  and  the  short-circuit  curve,  the  field 
current  required  to  produce  a  voltage  equal  to 
the  total  reactive  drop  of  the  circuit  (E\  sin 
Then 


ALTERNATING  CURRENTS  65 


when/    =the  field  current  required  to  produce   rated 

voltage  at  full-load. 
1 1  =the  field  current  required  to  produce  a  voltage 

equal  to  the  total  ohmic  drop  of  the  circuit. 
72  =  the  field  current  required  to  produce  a  voltage 

equal  to  the  total  reactive  drop  of  the  circuit. 

From  the  saturation  curve  find  the  open-circuit 
voltage  when  the  field  current  equals  /. 

6.  Graphical. — With  0  (Fig.  38)  as  a  center  and  a 
radius  proportional  to  the  rated  e.m.f.  strike  an  arc, 
ECD.     Through  0  draw  the  current  vector,  I.     Lay  off 
OA  proportional  to  the  resistance  drop,  RI,  and  AB 
proportional  to  the  reactance  drop,  XL     Draw  OC  to 
the  intersection  with  the  arc,  ECD,  so  that  the  cosine  of 
the  angle,   COI,  equals  the  power  factor  of  the  load 
circuit.     BC  is  proportional  to  the  generated  or  no- 
load  voltage. 

With  B  as  a  center  and  a  radius  equal  to  BC,  strike  a 
second  arc,  FCG.  Divide  OB  into  four  equal  parts  and 
lay  off  two  similar  divisions  beyond  0,  thus  obtaining 
points  representing  25  per  cent.,  50  per  cent.,  75  per  cent., 
100  per  cent.,  125  per  cent,  and  150  per  cent,  of  the  rated 
capacity.  From  these  points  and  from  B  draw  lines 
parallel  to  OC  to  their  intersection  with  the  arc,  FCG. 
The  lengths  of  these  parallel  lines  are  proportional  to  the 
terminal  voltages  at  the  given  percentages  of  load. 

7.  Efficiency. — The  efficiency  of  an  alternator  may  be 
determined  (a)  by  loading,  (b)  from  the  losses. 

(a)  Loading. — Load  the  alternator  and  determine 
the  input  and  the  output  for  loads  up  to  150 
per  cent,  of  the  rated  capacity. 

(b)  The  losses. — If  the  alternator  be  driven  by  a 
motor  or  other  mechanical  means,  the  losses  of 


66  DYNAMO  LABORATORY  OUTLINES  [2 

which  are  known  or  which  can  be  determined, 
the  losses  of  the  alternator  may  be  measured. 
These  losses  are  (1)  windage  and  friction,  (2) 
iron  (core)  loss,  (3)  field  copper  loss,  (4)  arma- 
ture copper  loss,  (5)  load  losses. 

Motor  Losses. — Disconnect  the  motor  and  the  al- 
ternator, run  the  motor  at  the  proper  speed  and  without 
load  and  determine  the  input  to  its  armature.  This 
input  supplies  the  stray  power  of  the  motor  which  is 
constant  (very  nearly)  and  a  small  copper  loss. 

Motor  stray  powerl  =  EI  —  Rmp 

(1)  Windage  and  Friction. — Drive  the  alternator 
at  rated  speed  but  without  field  excitation. 

Windage  and  friction  =  EI±  -Sm  —  Rmli2 

(2)  Iron  Losses. — Excite  alternator  field  to  give 
rated  voltage  at  ra,ted  speed. 

Iron  losses  =  EI2-Sm-Rml22-W&Fa 

(3)  Field    Copper    Loss. — From    the   saturation 
curve  find  the  field  current  (J3)  required  to 
give  rated  voltage  on  open  circuit. 

From  the  short-circuit  curve  find  the  current  (I^) 
required  to  give  any  desired  current  in  the  short-circuited 
armature  winding. 

Then  the  field  current  required  to  give  rated  voltage 
and  the  armature  current  at  the  same  time  is  the  vector 
sum  of  7  3  and  J4. 

Determine  the  resistance  (Rf)  of  the  field  winding. 

Field  copper  loss  =  (732+/42)#/ 

(Note. — The  loss  in  the  field  rheostat  should  not  be 
included  in  the  field  loss  as  the  rheostat  is  not  a  necessary 
part  of  the  apparatus.  See  A.I.E.E.  Rules.) 

1  See  Experiment  12  (Direct  Currents)  for  method  of  keeping 
the  stray  power  of  the  motor  constant. 


2]  ALTERNATING  CURRENTS  67 

(4)  Armature    Copper    Loss. — The    copper   loss 
of  the  armature  winding  is  readily  calculated 
for  any  value  of  armature  current  when  the 
resistance  of  the  winding  is  known. 

(5)  Load  Losses. — The  load   losses   include   all 
losses  not  included  in  (1),  (2),  (3)  and  (4),  are 
proportional  to  the  load  and  are  determined, 
for    any    given    armature    current,    from   the 
losses   of  the   alternator  when  the   armature 
winding  is  short-circuted  and  the  field  excitation 
is  such  that  the   given   current   flows   in  the 
short-circuited  winding. 

Load  losses  =  El,  -Sm-  Rmh2  -  W&Fa  -  Rah2 


Load  Loss 


These  losses  are  greater  when  the  armature  winding 
is  short-circuited  than  when  operating  under  load 
conditions.  Hence,  the  A.I.E.E.  Rules  recommend 


68  DYNAMO  LABORATORY  OUTLINES  [2 

that  one-third  the  loss,  as  determined  by     the   above 
method,  be  used  in  the  efficiency  calculations. 

8.  Determine  the   losses   of   an   alternator,   tabulate 
them  and  plot  as  in  Fig.  39. 

9.  Calculate 

(a)  the  efficiency  for  25  per  cent.,  50  per  cent., 
75  per  cent.,  100  per  cent.,  125  per  cent,  and  150 
per  cent,  of  the  rated  capacity. 

(6)  the  regulation  for  100  per  cent.,  80  per  cent, 
lagging  and  80  per  cent,  leading  power  factor  by 

(1)  the  e.m.f.  method. 

(2)  the  m.m.f.  method. 

(c)   the  synchronous  reactance  of  the  alternator. 

10.  Plot  the  following  curves: 

(a)  saturation. 

(b)  short-circuit. 

(c)  efficiency. 

(d)  voltage  characteristic. 

(1)  for  unity  power  factor. 

(2)  for  80  per  cent,  power  factor  leading. 

(3)  for  80  per  cent,  power  factor  lagging. 

11.  Explain 

(a)  why  the  e.m.f.   method  gives  a  poorer  regu- 
lation than  will  probably  be   obtained   by   an 
actual  load  test. 

(b)  why  the  m.m.f.  method  gives  a  better  regulation 
than  may  be  expected  on  load  test. 

(c)  why  the  voltage  changes  as  the  armature  cur- 
rent changes. 

(d)  the  meaning  of  the   terms  "synchronous   im- 
pedence"  and  "synchronous  reactance." 

(e)  why  a  considerable  variation  in  the  speed  while 


3]  ALTERNATING  CURRENTS  69 

taking  data  for  the  short-circuit  curve  is    of 
little  consequence. 

(f)  the  effect  of  the  power  factor  of  the  load  circuit 
on  the  rating  of  an  alternator. 

(g)  why  the  regulation  of  an  alternator  is  poorer 
when  the  load  is  inductive  than  when  it  is 
non-inductive. 

REFERENCES 

Franklin  &  Esty,  Alternating  Currents,  Chap.  7. 
Karapetoff,  Exp.  Elec.  Eng.,  Chap.  22. 
Karapetoff,  The  Magnetic  Circuit,  Chap.  8. 
Steinmetz,  A.-C.  Phenomena,  Chap.  22. 
Steinmetz,  Elements,  pp.  126-141. 
Bedell,  D.-C.  &  A.-C.  Testing,  Chap.  3. 
Smith,  Alternating  Currents,  Chap.  7. 


PARALLEL  OPERATION  OF  ALTERNATORS 

The  object  of  this  experiment  is  to  study  the  process 
of  connecting  two  alternating-current  generators  so 
that  they  will  supply  a  common  load  circuit,  to  determine 
the  division  of  the  load  between  them  and  the  effect  of 
unequal  field  excitation. 

1.  Connect  two  alternators  as  in  Fig.  40.     Regulate 
the  speeds  and  the  field  excitations  until  the  alterna- 
tors have  the  same  voltages  and  the  lamps  remain  dark 
for  several  seconds  at  a  time. 

If  all  the  lamps  are  not  dark  at  the  same  instant 
interchange  any  two  leads  from  the  same  machine. 
One  of  the  three  switches  may  now  be  closed. 

2.  During  a  dark  period  of  the  lamps  close  the  other 
two  switches  and  note  any  momentary  deflection  of  the 
ammeter  pointer. 

3.  Open  the  switches,  change  the  field  excitation  of 


70 


DYNAMO  LABORATORY  OUTLINES 


one  machine  by  10  per  cent,  to  25  per  cent,  and  close 
the  switches  as  before,  noting  the  momentary  and  the 
permanent  deflections  of  the  ammeter. 

Also  read  the  voltmeters  before  and  after  closing  the 
switches. 

4.  With  the  switches  closed,  change  the  field  excitation 
of  one  machine  and  note  the  changes  in  the  voltmeter 
and  the  ammeter  indications. 


FIG.  40. 

5.  Load  alternator  A  to  approximately  its  rated  capac- 
ity, synchronize  and  connect  alternator  B  to  the  load 
circuit. 

Regulate  the  driving  torque  of  machine  B  until  the 
alternators  divide  the  load  in  proportion  to  their  ratings. 

Increase  the  load  to  125  per  cent,  of  the  combined  ratings 
of  the  machines  and  note  the  division  of  the  load. 

6.  Reduce  the  excitation  of  one  of  the  machines  10 
per  cent,  to  25  per  cent.,  increase  that  of  the  other  so  that 


3]  ALTERNATING  CURRENTS  71 

the  voltage  remains  constant  and  note  the  load  division, 
the  total  output  being  kept  constant. 

7.  Reduce  the  driving  torque  of  one  machine  until  its 
wattmeters  indicate  approximately  zero,  disconnect  the 
driving  power  and  read  all  instruments.     (The  current 
leads  of  one  set  of  wattmeters  must  be  reversed,  indi- 
cating that  the  relation  of  the  current  and  the  e.m.f.  in 
this  portion  of  the  circuit. has  reversed  or  that  this  al- 
ternator is  now  running  as  a  motor.) 

8.  Vary  the  field  excitation  of  the  motor  and  note  the 
indications  of  the  instruments,  the  voltage  and  the  load 
being  kept  constant. 

9.  Increase  the  field  excitation  of  the  motor  until  one 
of  the  wattmeters  indicates  zero,  reverse  the  current  leads 
of  this  meter  and  increase  the  field  excitation  still  further, 
keeping  the  voltage  and  the  load  as  in  (8). 

(Note. — -The  load  in  (8)  and  (9)  may  be  applied  en- 
tirely or  in  part  by  loading  the  motor.) 

10.  Connect  a  synchroscope  and  use  it  in  place  of  the 
lamps. 

11.  Explain 

(a)  why  the  wattmeter  indications  are  practically 
constant  in  (6)  although  the  ammeter  indica- 
tions vary  greatly. 

(b)  why  a  momentary  current  flows  between   the 
machines  if  the  switches  are  closed  before  the 
lamps  become  entirely  dark. 

(c)  why  a  current  flows  between  the  machines  if  the 
field    excitation    of    one    machine    is    changed 
(switches  closed). 

(d)  why  the  indications  of  both  voltmeters  change 
when  the  excitation  of  one  machine  is  changed 
(switches  closed). 

(e)  by  means  of  a  diagram,  the  conditions  existing 
when  the  lamps  are  not  all  dark  at  the  same  time. 


72  DYNAMO  LABORATORY  OUTLINES  [4 

(/)    the  synchroscope  and  state  its  advantages. 

(g)  why  it  is  necessary  to  reverse  the  current  leads 
of  one  wattmeter  when  the  field  excitation  is 
increased  beyond  a  certain  value  as  in  (9). 
What  is  the  power  factor  of  the  motor  circuit 
when  one  wattmeter  indicates  zero?  How  is 
the  input  to  the  motor  found  when  measured 
by  the  two-wattmeter  method?  (Three-phase 
circuits.) 

(h)  why  the  wattmeters  measuring  the  input  to  the 
motor  (two  wattmeters  on  three-phase  circuit) 
do  not  read  the  same. 

(i)  by  means  of  a  diagram,  the  connections  so  that 
the  lamps  will  not  be  dark  at  synchronism. 

REFERENCES 

Franklin  &  Esty,  Alternating  Currents,  pp.  159-161. 

Karapetoff,  Exp.  Elec.  Eng.,  Chap.  25  &  Vol.  1,  pp.  357-363. 

Steinmetz,  A.-C.  Phenomena,  Chap.  23. 

Steinmetz,  Elements,  pp.  154-161. 

Bedell,  D.-C.  &  A.-C.  Testing,  pp.  146-149. 

Smith,  Alternating  Currents,  pp.  235-242. 

Standard  Handbook. 

Foster's  Handbook. 

4 

THE  SYNCHRONOUS  MOTOR 

The  object  of  this  experiment  is  to  study  the  alternator 
when  operated  as  a  motor. 

1.  Starting. — The  synchronous  motor,  as  such,  is  not 
self  starting  though  it  may  be  made  so  (if  polyphase)  by 
converting  it,  temporarily,  into  an  induction  or  hysteresis 
motor.  This  is  done  by  opening  the  field  circuit  and 
supplying  the  armature  with  a  reduced  voltage  by  means 
of  an  auto-transformer  or  other  starting  device.  Starting 


4]  ALTERNATING  CURRENTS  73 

in  this  way,  without  load,  the  rotor  will  attain  a  speed 
close  to  synchronism  (in  some  cases  greater  then  syn- 
chronism) when  the  field  switch  may  be  closed  and  the 
motor  will  drop  into  "step"  with  the  supply  system  and 
operate  as  a  synchronous  motor. 

(Warning. — In  starting  a  synchronous  motor  in  the 
above  manner,  a  dangerously  high  voltage  is  induced  in 
the  field  windings.  Great  care  should,  therefore,  be 
taken  to  avoid  contact  with  the  machine  while  starting.) 

The  synchronous  motor  may,  also,  be  started  by  means 
of  a  small  auxiliary  motor  (either  D.-C.  or  A.-C.) 
attached  to  the  shaft.  When  so  started,  it  acts  as  a 
generator  and  must  be  synchronized  (see  Parallel  Opera- 
tion of  Alternators)  before  the  switch  connecting  it  to  the 
alternating-current  system  is  closed.  After  connection 
is  made  to  the  ^alternating  current  system,  the  supply 
circuit  of  the  starting  motor  is  broken. 

2.  V-curves. — The   synchronous   motor   exhibits   the 
peculiarity  that  the  line  current  may  be  changed  by 
changing  the  excitation  of  the  D.-C.  field,  the  load  remain- 
ing  constant.     A   curve   plotted  with   line   current   as 
ordinates  and  field  current  as  abscissa  has  somewhat  the 
shape   of  the   letter   V — hence   the   name,    "V-curve." 
Starting  with  a  small  field  current,  the  line  current  de- 
creases to  a  minimum  and  then  increases  as  the  field 
excitation  is   increased.     The   point   of  minimum   line 
current  is  the  point  of  maximum  power  factor.     For 
field  excitations  below  this  the  current  is  lagging;  above, 
leading.     (See  Fig.  41.) 

3.  Clock  Diagram. — Draw  (Fig.  42)  OA  proportional 
to  the  applied  e.m.f.  and  let  OB  be  the  current  vector 
making  the  angle  AOB  with  the  applied  e.m.f.     At  right 
angles  to  OB  erect  OC  proportional  to  the  reactance 
drop,  XL     Draw  CD  parallel  to  OB  and  proportional  to 
the  drop  due  to  armature  resistance,  RI.     The  closing 


74 


DYNAMO  LABORATORY  OUTLINES 


[4 


line,  DA,  represents  the  motor  or  counter  e.m.f.,  while 
the  impedence  drop,  Z7,  is  represented  by  OD. 


Lagging    /    Leading 


Field    Amperes 

FIG.  41. 


FIG.  43. 


4.  Circle  Diagram.— If  the  vector,  OB  (Fig.  43)  be 
drawn  so  that  the  cosine  of  the  angle,  A  OB,  equals  the 
power  factor  of  the  motor  when  the  rotor  is  blocked  and 
OB  is  made  proportional  to  the  applied  e.m.f.,  the  locus 


4]  ALTERNATING  CURRENTS  75 

of  the  current  vector  is  a  semi-circle  whose  center  is  at  B 
and  whose  radius  is  proportional  to  the  field  excitation, 
i.e.,  to  the  counter  e.m.f.  Then  for  a  given  field  excita- 
tion or  counter  e.m.f.  the  complete  diagram  for  any  load 
may  be  determined. 

5.  Efficiency. — The  efficiency  of  a  synchronous  motor 
is  determined  (a)  by  a  brake  test,  the  input  and  the 
output  being  measured;  (b)  from  the  losses  calculated  as 
for  an  alternator. 

6.  Construct  the  following  curves: 

(a)  efficiency. 

(6)  V-curves  for  not  less  than  three  different  loads. 

(c)  vector  (clock)  diagram. 

(d)  circle  diagram. 

7.  Explain 

(a)  how  the  power  factor  changes  with  the  load,  the 
field  excitation  remaining  constant. 

(b)  how  the  shape  of  the  V-curve  affects  the  opera- 
tion of  the  motor. 

(c)  why  the  field  circuit  is  opened  before  starting 
as  an  induction  motor. 

(d)  why    the    branches    of    the    V-curve    are    not 
symmetrical. 

(e)  how  the  counter  e.m.f.  may  be  greater  than  the 
applied  e.m.f. 

(/)    the  effect  of  reversing  the  field  current  while  the 

motor  is  in  operation. 
(g)  the  commercial  applications  of  the  synchronous 

motor. 
(h)  by   means    of   a    diagram,  how   the    armature 

current  (the  load)  increases,  the  speed  remaining 

constant. 
(i)    "  hunting,"  its  cause  and  remedy. 


76  DYNAMO  LABORATORY  OUTLINES  [5 


REFERENCES 

Franklin  &  Esty,  Alternating  Currents,  Chap.  8. 
Karapetoff,  Exp.  Elec.  Eng.,  Chap.  21. 
Steinmetz,  A.-C.  Phenomena,  Chap.  24. 
Steinmetz,  Elements,  pp.  141-154. 
McAllister,  A.-C.  Motors,  Chap.  10. 
Bedell,  D.-C.  &  A.-C.  Testing,  Chap.  2. 
Smith,  Alternating  Currents,  Chap.  8. 
Standard  Handbook. 
Foster's  Handbook. 


THE  ROTARY  CONVERTER 

The  object  of  this  experiment  is  to  study  the  rotary 
or  synchronous  converter. 

1.  Starting. — When  supplied  with  direct  current  the 
converter  acts  as  a  D.-C.  motor  and  may  be  started  as 
such  by  the  use  of  the  usual  "starting  box. "     Before 
connection  is  made  to  an   alternating-current  system, 
correct  voltage  and  phase  relations  must  be  obtained. 
(See  Parallel  Operation  of  Alternators.) 

When  supplied  with  alternating  current  the  converter 
is  not  self  starting  but  may  be  started  by  induction 
(hysteresis)  motor  action  as  described  for  the  synchronous 
motor.  (See  The  Synchronous  Motor.) 

2.  Run  the  converter  as  a  D.-C.  motor,  measure  the 
speed,  the  applied  voltage  and  the  voltage  at  the  rings. 

3.  Change  the  field  excitation  and  repeat  (2). 

4.  Apply  a  load  to  the  A.-C.  side  and  repeat  (2). 

5.  Change  the  field  excitation  and  repeat  (4). 

6.  Synchronize  the  A.-C.  side  with  an  A.-C.  system 
of  the  proper  voltage  and  frequency  and  note  the  effect 
of  a  change  in  field  excitation  on  the  speed  of  the  rotary 
and  on  the  current  output,  the  load  being  kept  constant. 


6]  ALTERNATING  CURRENTS  77 

7.  Start  as  an  induction   (hysteresis)   motor,  noting 
the   current  intake  before  and  after  closing  the  field 
" break-up  switch." 

8.  Run  the  converter  as  a  synchronous  motor,  measure 
the  speed,  the  applied  voltage  and  the  voltage  at  the 
D.-C.  brushes. 

9.  Change  the  field  excitation  and  repeat   (8),  also 
observing  any  change  in  the  A.-C.  ammeter  indication. 

10.  Compounding. — The  D.-C.  voltage  of  a  converter 
may  be  increased  with  the  load  by  means  of  a  series 
field  winding  provided  there  is  inductance  in  the  A.-C. 
supply  circuit.     The  inherent  inductance  in  the  A.-C. 
system  may  be  sufficient  for  this  purpose  or  it  may  be 
introduced  in  the  form   of  an  auto-transformer,  a  re- 
actance coil,  or  otherwise. 

With  the  converter  taking  power  from  an  A.-C.  system 
and  the  D.-C.  field  provided  with  a  compound  winding, 
load  the  converter  and  note  the  change  in  voltage  at  the 
rings  and  at  the  D.-C.  brushes,  the  voltage  of  the  supply 
circuit  being  kept  constant.  (Note. — If  no  compound 
converter  is  available,  the  effect  of  an  increased  field 
excitation  may  be  observed  by  increasing  the  shunt 
field  current  by  means  of  the  field  rheostat.) 

11.  V-curves. — The  converter  has  the  same  charac- 
teristic as  a  synchronous  motor  in  that,  with  a  given 
load,  the  A.-C.  line  current  is  a  minimum  for  a  certain 
field  current.     For  field  currents  less  than  this,  the  line 
current  increases  in  value  and  lags  behind  the  e.m.f. 
For  greater  field  excitations,  the  line  current  increases 
but  leads  the  e.m.f. 

12.  Efficiency. — The    efficiency    of    a    converter    is 
determined  (a)  by  loading,  (6)  from  the  losses. 

(a)  Loading. — Load    the    converter    and    measure 

the  input  and  the  output. 
(6)  The   losses. — The   only   measurements   needed 


78  DYNAMO  LABORATOEY  OUTLINES  [6 

are  the  no-load  input  at  rated  speed  and  voltage, 
measured  from  either  the  A.-C.  or  the  D.-C. 
side,  and  the  resistance  between  the  D.-C. 
brushes  as  determined  by  the  voltmeter-ammeter 
or  other  D.-C.  method. 
Then 

(EI-RP)IOO 
per  cent,  efficiency  = ^,,      '  — 

when  E  =  ihe  rated  D.-C.  voltage. 
1  =  the  rated  D.-C.  current. 

TF  =  the  no-load  input  in  watts. 

jR  =  the  effective  armature  resistance  which 
is  proportional  to  the  resistance  as  measured 
between  the  D.-C.  brushes  and  depends  on  the 
number  of  rings  on  the  A.-C.  side.  To  obtain 
the  value  of  R  multiply  the  measured  resistance 
by  the  following: 

For  a  2-ring  converter,  1.39 

For  a  3-ring  converter,  0.56 

For  a  4-ring  converter,  0.37 

For  a  6-ring  converter,  0.26 

For  an  8-ring  converter,  0.21 

13.  With  current  output  as  abscissa,  plot  the  following 
curves : 

(a)  D.-C.  voltage 

(b)  A.-C.  voltage. 

(c)  watts  input. 

(d)  watts  output. 

(e)  losses. 

(/)    efficiency. 

14.  Explain 

(a)  the  effect  of  a  change  in  field  excitation  (1) 


5]  ALTERNATING  CURRENTS  79 

when  the  converter  is  operated  inverted  and 
in  parallel  with  synchronous  generators;  (2) 
when  operated  inverted  but  independent  of 
synchronous  machines;  (3)  when  delivering 
direct  current. 

(6)  the  effect  should  the  A.-C.  system  be  discon- 
nected from  the  converter  when  the  converter 
is  connected  to  a  D.-C.  system  and  operating  with 
a  weak  field.  What  precaution  should  be  taken 
to  provide  for  such  an  emergency? 

(c)  the  " split-pole"  converter. 

(d)  the  advantages  and  the  disadvantages  of  the  con- 
verter compared  with  other  methods  of  conver- 
sion. 

(e)  the  increase  in  the  capacity  of  a  given  armature 
as  the  number  of  rings  increases. 

(/)  how  the  presence  of  inductance  in  the  A.-C.  cir- 
cuit makes  compounding  possible. 

(g)  why  the  effective  resistance  is  different  from 
the  resistance  as  measured  between  the  D.-C. 
brushes. 

(h)  why  there  is  no  need  to  shift  the  brushes  as  the 
load  varies,  as  in  the  case  of  the  D.-C.  dynamo. 

(i)  why  the  theoretical  and  the  observed  ratios  of 
A.-C.  to  D.-C.  voltage  are  not  the  same. 

REFERENCES 

Franklin  &  Esty,  Alternating  Currents,  Chap.  9. 
Karapetoff,  Exp.  Elec.  Eng.,  Chap.  23. 
McAllister,  A.-C.  Motors,  Chap.  10. 
Smith,  Alternating  Currents,  Chap.  10. 
Steinmetz,  Elements,  pp.  217-160. 
Standard  Handbook. 
Foster's  Handbook. 


80  DYNAMO  LABORATORY  OUTLINES  [6 


THE  INDUCTION  MOTOR 

The  object  of  this  experiment  is  to  study  the  induction 
motor  and  to  obtain  data  from  which  curves  representing 
the  actions  of  such  a  motor  may  be  plotted. 

1.  Induction  motors  are  divided  into  two  classes  differ- 
entiated, in  construction,  by  the  rotor  winding. 

(a)  The  squirrel-cage  motor  has  a  rotor  whose  con- 
ductors are  insulated  copper  wires  or  bars  placed 
in  slots  on  a  cylindrical,  laminated  iron  core,  the 
ends  of  the  conductors  being  connected  by  copper 
rings,  one  on  each  end  of  the  core. 

(b)  The  wound  rotor  is  one  in  which  distinct  wind- 
ings are  interconnected  and  the  proper  terminals 
brought  out  to  slip  rings  mounted  on  the  shaft. 
Through  these  slip  rings  connection  is  made  to  a 
rheostat  by  means  of  which  the  resistance  of  the 
rotor  circuit  may  be  varied. 

2.  Starting. — The  induction  motor  is  self  starting  (if 
polyphase)  when  supplied  with  alternating  current  of  the 
proper  frequency,  voltage  and  number  of  phases.     The 
squirrel-cage  motor  is  usually  started  by  supplying  the 
stator  with  a  voltage  less  than  that  at  which  the  motor 
is  rated,  the  line  voltage  being  reduced  by  means  of  an 
auto-transformer  or  other  step-down  device.     After  the 
rotor  attains  a  considerable  speed,  the  line  voltage  is 
applied  and  the  starting  device  automatically  discon- 
nected from  the  line. 

In  the  wound-rotor  type,  rated  voltage  is  supplied  to 
the  stator,  resistance  having  been  introduced  into  the 
rotor  circuit.  As  the  rotor  speeds  up,  the  resistance  in 
the  rotor  circuit  is  reduced. 


6] 


ALTERNATING  CURRENTS 


81 


3.  Performance  Curves. — The  performance  of  an  in- 
duction motor  is  shown  by  curves  as  in  Fig.  44,  data  for 
which  may  be  derived  from  (a)  a  brake  test,  (b)  the  losses, 
(c)  the  circle  diagram. 

(a)  Brake  test. — Supply  the  motor  from  a  circuit  of 
the  rated  voltage  and  frequency,  and  determine 
the  following  quantities  for  loads  up  to  125  per 
cent,  of  the  rated  capacity: 


Synchronous  Speed 


Horse  Power  Output 
FlG.  44. 

(1)  watts  input. 

(2)  voltage. 

(3)  current. 

(4)  torque. 

(5)  speed. 

(6)  slip. 

(6)  The  losses. — The  set-up  for  the  determination 
of  the  losses  is  the  same  as  that  for  the  brake 
test  except  that  no  torque  measurements  need 


82  DYNAMO  LABORATORY  OUTLINES  [6 

be  taken,  the  motor  being  connected  to  an  elec- 
tric generator  or  other  apparatus  by  means  of 
which  it  may  be  loaded. 

At  no-load  the  input  is  composed  of  the  different  motor 
losses — stray  power  (iron  loss,  windage  and  friction), 
stator  copper  loss  and  rotor  copper  loss.  The  stator 
copper  loss  is  readily  computed  from  the  current  input 
and  the  measured  resistance  of  the  stator  windings.  In 
the  squirrel-cage  rotor  the  copper  losses  in  the  rotor  wind- 
ing are  small  and  may  be  neglected  while  in  the  wound 
rotor  they  are  determined  as  readily  as  in  the  stator. 
Then 

Stray  power  =  no-load  input  —  copper  losses. 

For  any  input 

Output  =  input  —  stray  power  — stator  PR  —  rotor  PR. 
But  the  rotor  copper  losses  are  porportional  to  the  slip. 
Hence 

,.  TOT*\  100—  per  cent,  slip 

Output  =  (input  —stray  power  —  stator  72R) —          inn — 

luu 

The  slip  of  an  induction  motor  is  easily  measured, 
unless  it  becomes  excessive,  by  the  stroboscopic  method. 
On  the  end  of  the  shaft  or  pulley  mark  as  many  equally 
spaced  radial  lines  as  there  are  pairs  of  poles  on  the  motor. 
Strongly  illuminate  these  marks  by  means  of  an  arc 
lamp  supplied  from  the  same  source  as  the  motor.  When 
the  motor  is  in  operation,  the  radial  lines  appear  to  rotate 
in  a  direction  opposite  to  that  of  the  rotor.  The  speed 
of  this  apparent  rotation  is  proportional  to  the  slip  of  the 
rotor. 

Another  simple  method  for  the  determination  of  slip 
is  to  connect  a  contact  maker  to  the  shaft  of  the  motor 
so  that  it  will  close,  once  in  each  revolution,  the  circuit 
of  a  D.-C.  voltmeter  connected  across  the  circuit  supply- 


6]  ALTERNATING  CURRENTS  83 

ing  the  motor.  The  voltmeter  pointer  will  Wing  back 
and  forth,  the  rate  of  swing  being  porportional  to  the 
slip  of  the  rotor. 

(c)  The  circle  diagram. — By  means  of  the  circle 
diagram  the  performance  of  an  induction  motor 
is  determined  from  two  simple  tests  and  the 
resistance  of  the  stator  winding. 

No-load  Test. — Run  the  motor  without  load,  at  its 
rated  voltage  and  measure  current  and  watts  input. 

Blocked  Rotor  Test. — Block  the  rotor  to  prevent  its 
rotation,  apply  rated  voltage  and  measure  current  and 
watts  input. 


Volts 

FIG.  45. 

(Note.  —  It  is  often  inadvisable  to  apply  rated  voltage 
to  a  motor  with  the  rotor  blocked  because  of  the  large 
currents  that  will  flow  in  the  windings.  If  several 
wattmeter  readings  be  taken  at  voltages  less  than  the 
rated  voltage  of  the  motor,  a  curve  may  be  plotted 

between  —        and  volts   (Fig.  45).     This   curve  is  a 


straight  line  and  may  be  extended  to  any  desired  point, 
the  ordinate  of  which,  when  multiplied  by  the  abscissa, 
will  give  the  watts  input  at  that  voltage.  This  makes  it 
unnecessary  to  cause  excessive  currents  to  flow  in  the 


84 


DYNAMO  LABOKATORY  OUTLINES 


[6 


motor  windings,  but  very  low  voltages  should  not  be 
used,  as  the  results,  with  such  voltages,  are  likely  to  be 
erratic. 

Construction  of  the  Circle  Diagram. — Draw  OA  (Fig. 
46)  proportional  to  the  rated  e.m.f.  From  0  lay  off  OB 
proportional  to  the  no-load  current,  the  cosine  of  the 
angle,  AOB,  being  equal  to  the  power  factor  at  no-load. 
From  B  draw  the  line,  BD,  parallel  to  the  axis  of  abscissa. 
Also  from  0  lay  off  the  line,  OC,  proportional  to  the 


z   KH 


FIG.  46. 


current  with  the  rotor  blocked,  the  cosine  of  the  angle, 
AOC,  being  equal  to  the  power  factor  under  this  condi- 
tion. The  points,  B  and  C,  are  two  points  on  the  semi- 
circular locus  of  the  current  vector,  the  center  being  on 
the  line,  BD. 

The  perpendicular  to  BD  dropped  from  C  is  propor- 
tional to  the  copper  loss  with  rotor  blocked.  From  the 
measured  resistance  of  the  stator  winding  and  the 
ammeter  reading  the  copper  loss  of  the  stator  winding 
is  calculated  and  laid  off  proportional  to  EF.  Draw 
straight  lines  from  C  and  F  to  B.  Then,  for  any  stator 


6]  ALTERNATING  CURRENTS  85 

current,  as  OG,  the  no-load  losses  (constant)  are  propor- 
tional to  HK,  the  stator  copper  loss  to  KL,  the  rotor 
copper  loss  to  LM  and  the  motor  output  to  MG.  For 
any  other  value  of  current  input,  the  relations  may  be 
found  in  a  similar  manner. 

Power  Factor. — With  0  as  a  center  and  any  convenient 
radius,  draw  the  arc,  NP.  The  ratio  of  the  projection 
of  the  current  vector  (produced  if  necessary)  between  0 
and  the  intersection  with  the  quadrant  on  OP  to  OP  is  the 
power  factor  of  the  motor  for  that  current  input. 

The  maximum  power  factor  at  which  an  induction 
motor  will  operate  is  obtained  when  the  current  vector 
is  tangent  to  the  circle. 

Slip. — Through  the  point,  T,  on  BC  (extended)  draw 
a  line  parallel  to  BF  and  intersecting,  at  S,  a  perpen- 
dicular to  the  axis  of  abscissa  erected  at  B.  Divide  the 
line,  ST,  into  100  equal  parts,  beginning  at  S.  Draw  a 
line  from  B  through  the  end  of  the  current  vector  to  its 
intersection  with  ST.  The  slip  is  read  directly  from 
the  scale  on  ST. 

Efficiency. — Parallel  to  the  axis  of  abscissa  draw  the 
line,  XY,  to  its  intersection  with  BC  (extended).  Also 
extend  BC  to  its  intersection  with  the  axis  of  abscissa 
and  erect  the  perpendicular,  XZ.  Divide  XY  into  100 
equal  parts,  beginning  at  Y.  Through  Z  and  the  end 
of  the  current  vector  draw  a  line,  extending  it  to  its 
intersection  with  XY.  The  efficiency  is  read  directly 
from  the  scale,  XY. 

Maximum  Output. — The  maximum  output  of  the 
motor  is  at  that  point  where  a  line  passing  through  the 
end  of  the  current  vector  and  tangent  to  the  circle  is 
parallel  to  BC. 

4.  Balancing. — When  a  polyphase  motor  is  operated 
on  a  badly  unbalanced  system,  there  is  a  "  balancing" 
effect  which  tends  to  equalize  the  system. 


86  DYNAMO  LABORATORY  OUTLINES  [6 

5.  Cascade. — The  speed  of  an  induction  motor  may 
be  reduced  by  increasing  the  resistance  of  the  rotor 
circuit  but  this  reduces  the  efficiency.     If  the    rotor 
circuit  be  used  to  supply  the  stator  circuit  of  a  second 
motor  instead  of  being  dissipated  in  a  rheostat,   a  re- 
duction  of  speed  is   accomplished  without  greatly  re- 
ducing the  efficiency. 

Connect  the  rotor  windings  of  a  wound  rotor  to  the 
stator  windings  of  another  induction  motor  (wound  or 
squirrel-cage)  through  an  auto-transformer  (if  necessary, 
to  get  the  proper  voltage  on  the  second  machine)  and 
note  the  speed  and  the  load  division  when  the  shafts 
are  tied  together  mechanically. 

6.  Frequency  Changer. — When  the  rotor  of  an  in- 
duction motor  is  driven  at  a  speed  less  than  synchronism, 
the  motor  acts  both  as  a  transformer  and  as  a  generator, 
the  frequency  of  the  rotor  circuit  depending  on  the  speed 
of  the  rotor. 

(a)  Drive  the  rotor  of  a  wound  motor  at  various 
speeds  from  synchronism  to  synchronous  speed 
backward  and  observe  voltages  and  frequencies. 

(6)  Drive  the  rotor  of  a  wound  motor  at  such  a  speed 
as  to  obtain  some  desired  frequency.  Keep 
this  frequency  constant,  load  the  rotor  circuit 
by  means  of  a  water  rheostat,  motor  or  other- 
wise, and  measure  the  following  quantities : 

(1)  watts  input  to  stator  of  motor. 

(2)  watts  input  to  driving  motor. 

(3)  watts  output. 

7.  Single-phase   Induction   Motor. — The    theory    of 
the  single-phase  induction  motor  is  rather  complicated 
because  of  the  irregular  form  of  the  current  and  flux 
waves.     What  has  been  said  above  is,  however,  applicable 
to  the  single-phase  motor  with  only  slight  modifications. 


6] 


ALTERNATING  CURRENTS 


87 


(a)  Using  a  single-phase  motor  (with  an  auxiliary 
starting  phase)  or  a  three-phase  motor  connected 
as  in  Fig.  47  determine 

(1)  maximum  line  current. 

(2)  current  in  running  phase. 

(3)  current  in  starting  phase. 

(4)  time  required  for  motor  to  reach  full  speed. 
(6)   Determine  the  value  of  x  and  of  r  to  give 

(1)  minimum  starting  current. 

(2)  minimum  time  to  reach  full  speed. 


Single  Phase  Supply 


Inductance 


Resistance 


FIG.  47. 

8.  Determine,  for  an  induction  motor, 

(a)  the  line  disturbances  caused  in  starting. 
(6)  the  resistance  of  the  stator  winding. 

(c)  the  " pull-out"  torque  in  per  cent,  of  full-load 
torque. 

(d)  the  voltage  required  to  give  maximum  starting 
torque. 

(e)  the  slip  by  direct  measurement. 
(/)    the  stray  power. 


88  DYNAMO  LABORATORY  OUTLINES  [6 

(g)  the  stator  copper  loss. 

(h)  the  rotor  copper  loss. 

(i)    the  maximum  output. 

(k)  the  maximum  power  factor. 

(I)    the  efficiency  at  25  per  cent.,  50  per  cent.,  75 

per  cent.,  100  per  cent.,  125  per  cent,  and  150  per 

cent,  of  full-load. 

9.  Construct,  for  an  induction  motor, 

(a)  the  circle  diagram. 

(6)  the  performance  curves. 

(1)  speed. 

(2)  power  factor. 

(3)  efficiency. 

(4)  torque. 

(5)  slip. 

using  horse-power  output  as  abscissa. 

10.  Explain 

(a)  the  relation  between  slip  and  losses.     (Consider 

both  single-  and  polyphase  motors.) 
(6)  the  relation  between  slip  and  load  or  torque. 

(c)  the  effects  when  a  polyphase  motor  is  operated 
on  an  unbalanced  system. 

(d)  the  meaning  of  "synchronous  watts"  or  "syn- 
chronous horse-power." 

(e)  the  relation  between  slip  and  the  frequency  of 
the  rotor  circuit. 

(/)    the  relation  between  slip  and  the  voltage  of  the 

rotor  circuit. 
(g)  the  relation  between  slip  and  applied  voltage, 

the  load  remaining  constant. 
(h)  the  relation  between  frequency  and  speed  in  a 

frequency  changer. 
(i)    the  relation  between  the  frequency  of  the  supply 


7]  ALTERNATING  CURRENTS  89 

system,  that  of  the  load  circuit  and  the  size  of  a 
frequency  changer  and  its  driving  motor. 

(k)  the  relation  between  the  speed  of  the  rotor  and 
the  e.  m.  f.  of  the  rotor  circuit  in  a  frequency- 
changer  set. 

(I)  the  measurement  of  slip  by  the  methods  out- 
lined in  (3). 

(ra)  the  cause  of  the  large  starting  current  in  the 
squirrel-cage  motor'. 

REFERENCES 

Franklin  &  Esty,  Alternating  Currents,  Chap.  12-13. 
Karapetoff,  Exp.  Elec.  Eng.,  Chap.  24-25. 
McAllister,  A.-C.  Motors. 
Bailey,  The  Induction  Motor. 
Steinmetz,  Elements,  pp.  261-297,  315-320. 
Smith,  Alternating  Currents,  Chap.  11—12. 
Foster's  Handbook. 
Standard  Handbook. 


THE  INDUCTION  GENERATOR 

The  object  of  this  experiment  is  to  study  the  action 
of  the  induction  motor  when  run  above  synchronism. 

1.  Make  connections  as  shown  in  Fig.  48.     Regulate 
the  speed  and  the  field  excitation  of  the  alternator  to  give 
rated  frequency  and  voltage.     Close  the  switch,  Si,  and 
start  the  induction  motor  in  the  usual  way.     Speed  up 
the  motor,  A,  so  as  to  drive  the  induction  motor  above 
synchronism.     Open  the  supply  circuit  of  motor,  J9,  and 
regulate  the  speed  of  the  induction  motor  and  the  excita- 
tion of  the  synchronous  machine  so  that  the  frequency 
and  the  voltage  of  the  system  are  normal.     Tabulate  the 
readings  of  the  instruments. 

2.  Close  switch,  Sz,  and  take  readings  for  different  out- 
puts, keeping  the  speed  and  the  field  excitation  constant. 


90 


DYNAMO  LABORATORY  OUTLINES 


[7 


3.  Repeat  (2)  keeping  frequency  and  field  excitation 
constant. 

4.  Repeat  (2)  keeping  frequency  and  voltage  constant. 

5.  With  output  as  abscissa,  plot  curves  with  the  follow- 
ing ordinates : 


FIG.  48. 

(a)  frequency. 
(6)  voltage. 

(c)  speed. 

(d)  excitation. 

6.  Explain 

(a)  the  relation  between  speed  and  frequency. 
(6)  the  relation  between  voltage  and  excitation. 


8]  ALTERNATING  CURRENTS  91 

(c)  the  relation  between  speed  and  voltage. 

(d)  the  relation  between  speed  and  load. 

(e)  the  relation  between  voltage  and  load. 
(/)    the  power-factor  relations. 

(g)  why  it  is  not  necessary  to  synchronize  the  in- 
duction generator  before  connecting  it  to  an 
A.-C.  system. 

REFERENCES 

Franklin  &  Esty,  Alternating  Currents,  p.  271. 
McAllister,  A.-C.  Motors,  Chap.  7. 
Steinmetz,  A.-C.  Phenomena,  pp.  310-319. 
Steinmetz,  Elements,  pp.  291-307. 
Bailey,  The  Induction  Motor,  Chap.  5. 

8 

THE  SINGLE-PHASE  COMMUTATING  MOTOR 

The  object  of  this  experiment  is  to  study  the  single- 
phase  commutating  motor  and  to  obtain  data  from 
which  to  plot  the  performance  curves. 

1.  Connect   the   field   and   armature   windings   of   a 
series  A.-C.  commutating  motor  to  A.-C.  mains  of  the 
rated  voltage  and  frequency.     Short-circuit  the  compen- 
sating winding.     Determine  the  following: 

(a)  watts  input. 

(b)  current. 

(c)  speed. 

(d)  torque. 

(e)  output. 

(f)  power  factor. 

(g)  efficiency. 

2.  Repeat   (1)   with  the  compensating  winding,   the 
field  winding  and  the  armature  winding  in  series. 


92  DYMAMO  LABORATOKY  OUTLINES  [8 

3.  Repeat  (1)  with  the  compensating  winding  open. 

4.  Repeat  (1)  with  the  armature  winding  disconnected 
from  the  supply  circuit  and  the  brushes  short-circuited. 

5.  Repeat  (4)  with  the  compensating  winding  open. 

6.  Repeat  (1),  (2)  and  (3),  using  D.C.  of  such  a  voltage 
that  the  current  does  not  become  excessive. 

7.  Test    other    types    of    single-phase    commutating 
motors. 

8.  Using   horse-power   output   as   abscissa,    plot   the 
following  curves: 

(a)  current. 

(b)  power  factor. 

(c)  speed. 

(d)  torque. 

(e)  efficiency. 
(/)    input. 

9.  Explain 

(a)  the  action  of.  a  D.-C.  shunt  motor  when  supplied 
with  single-phase  alternating  current. 

(6)  the  action  of  a  D.-C.  shunt  motor  when  the 
armature  is  supplied  with  current  from  one 
phase  and  the  field  with  current  from  the  other 
phase  of  a  two-phase  system. 

(c)  the  structural  differences  in  the  A.-C.  and  the 
D.-C.  series  motor. 

(d)  the  action  of  the  compensating  winding. 

(e)  the  action  of  the  series  motor  when  the  brushes 
are  short-circuited. 

(/)  how  excessive  sparking  in  the  A.-C.  series  motor 
is  prevented. 

10.  Compare    the    starting    torques    for    the    same 
armature  current  when  the  same  motor  is  operated  on 
A.C.  and  on  D.C. 


9]  ALTERNATING  CURRENTS  93 

REFERENCES 

Franklin  &  Estey,  Alternating  Currents,  Chap.  14. 
McAllister,  A.-C.  Motors,  Chap.  12-15. 
Steinmetz,  A.-C.  Phenomena,  Chap.  27. 
Bailey,  The  Induction  Motor,  Chap.  14. 
Smith,  Alternating  Currents,  Chap.  12. 
Standard  Handbook. 
Foster's  Handbook. 


9 

THE  CONSTANT  POTENTIAL  TRANSFORMER 

The  object  of  this  experiment  is  to  study  the  constant 
potential  transformer  and  to  determine  the  losses,  the 
regulation,  the  efficiency  and  the  heating. 

1.  The  Losses. — The  losses  in  a  transformer  are  (a) 
iron  losses,  (6)  copper  losses. 


FIG.  49. 

(a)  Iron  losses. — Connect  the  transformer  as  in 
Fig.  49  and  impress  on  it  voltages  varying  from 
20  per  cent,  to  150  per  cent,  of  the  rated  e.m.f., 
the  frequency  being  kept  constant.  The  watt- 
meter will  indicate  the  iron  loss  plus  a  small 
copper  loss.  Since  the  copper  loss  is  that  due  to 
the  small  no-load  current,  it  may  usually  be 
neglected. 

Tabulate  not  less  than  six  readings  of  watts  and  volts. 

(6)  Copper  losses. — Connect  the  transformer  as  in 
Fig.  50  and  impress  on  one  of  the  windings 


94 


DYNAMO  LABORATORY  OUTLINES 


[9 


(preferably  the  high-tension)  such  voltages  as 
will  cause  the  current  to  vary  from  zero  to  150 
per  cent,  of  full-load  (rated)  value.  The  watt- 
meter will  indicate  the  copper  loss  plus  a  small 
iron  loss  which  is  neglected. 


/TN 

0 

o 

§ 

o 

o 

0 
O 

o 

xKT       VfV 

itr 

i 

FIG.  50. 

Tabulate  the  wattmeter,  voltmeter  and  ammeter 
readings  for  not  less  than  six  values  of  current. 

(Note. — This  will  require  only  a  small  percentage  of 
the  normal  voltage  and  care  should  be  taken  that  too  large 
a  voltage  is  not  applied  or  an  excessive  current  may  flow 
and  the  transformer  or  the  instruments  be  damaged.) 


Volts  Amperes 

FIG.  51. 

2.  Efficiency. — The  efficiency  of  a  transformer  is 
determined  (a)  by  loading,  (6)  from  the  losses  as  found  in 
(1)  above,  (c)  by  an  opposition  test. 

(a)  Loading. — Supply  the  transformer  with  current 
at  the  rated  e.m.f.  and  measure  the  input  and 


9] 


ALTERNATING  CURRENTS 


95 


the  output  for  loads  from  zero  to  150  per  cent, 
of  the  rated  load. 

(6)  From  the  losses. — From  the  iron-loss  curve 
(Fig.  51)  the  core  loss  for  the  rated  e.m.f.  may 
be  obtained.  This  loss  is  practically  constant 
for  varying  values  of  current  so  long  as  the 
applied  voltage  is  constant. 


,1 

,1 

UMmm 


[0000  JOOOOOOOOOI 


FIG.  52. 

The  copper  loss  varies  as  the  square  of  the  current  and 
its  value  for  any  load  current  may  be  obtained  directly 
from  the  copper-loss  curve  (Fig.  51). 

(c)  Opposition  test. — Connect  two  identical  trans- 
formers as  in  Fig.  52  (protecting  them  by 
suitable  fuses  or  circuit  breakers)  to  a  circuit  of 
the  rated  voltage  and  frequency. 

Regulate  the  e.m.f.  applied  to  the  transformer,  773,  and 
take  wattmeter  readings  for  current  values  varying  from 


96  DYNAMO  LABORATORY  OUTLINES  [9 

zero  to  150  per  cent,  of  rated  current  as  indicated  by  the 
ammeter.  The  wattmeters  will  indicate  the  losses  of  the 
two  transformers  under  load  conditions. 

(Note.  —  Disconnect  the  voltmeter  leads  and  the  connec- 
tions to  the  potential  coils  of  the  wattmeters  before 
opening  or  closing  the  supply  circuits.) 

3.  Regulation.  —  The  regulation  of  a  transformer  is 
calculated  from  (a)  loading,  (6)  the  losses. 

(a)  Loading.  —  Determine  the  secondary  voltage 
at  no-load  and  at  full-load,  keeping  the  primary 
e.m.f.  constant  at  its  rated  value. 

(6)   From  the  losses.  —  From  the  data  obtained  in 

(lb~)  or  (2c) 

W  = 
W 


when          R  =  the  equivalent  resistance  of  both  coils. 
Z  =the    equivalent    impedence    of  both 

coils. 

X  =  the  equivalent  reactance  of  both  coils. 
W  =  the  copper  loss  in  the  transformer. 


then  Eo  =  \/(El  cos  c/>+RI)2+(El  sin 

when          E0  =  the  no-load  voltage. 

EI  =  the  rated  voltage. 
cos  (j>  =the  power  factor  of  the  load  circuit. 

4.  Kapp's  Diagram.  —  The  regulation  of  a  transformer 
is  determined  graphically  by  means  of  Kapp's  diagram 
(Fig.  53). 

With  0  as  a  center  and  a  radius  proportional  to  the 
rated  primary  e.m.f.,  describe  a  semicircle.  On  the 


9] 


ALTERNATING  CURRENTS 


97 


diameter,  DE,  lay  off  OA  proportional  to  XI,  the 
reactance  drop  at  full-load.  At  right  angles  to  DE  erect 
the  current  vector,  7,  and  lay  off  A  B  proportional  to  the 
full-load  resistance  drop,  RI.  From  B  draw  the  vector, 
BC,  to  the  intersection  with  the  semicircle  and  making 
the  angle,  <f>,  whose  cosine  is  the  power  factor  of  the  load 


FIG.  53. 

circuit,  with  the  current  vector.     BC  is  proportional  to 
the  secondary  voltage  reduced  to  the  primary  circuit. 

5.  Heat  Test. — The  rise  in  temperature  due  to  the 
losses  limits  the  output  of  a  given  transformer.  This 
rise  in  temperature  is  determined  (a)  by  loading,  (6) 
from  an  opposition  test. 

(a)  Loading. — After  determining  the  resistance  of 
the  windings,  operate  the  transformer  at  rated 
voltage  and  full-load  current.  Take  periodic 


98 


DYNAMO  LABORATORY  OUTLINES 


[9 


(6) 


readings  of  thermometers  placed  in  the  oil  of  the 
transformer  and  measure,  at  regular  intervals, 
the  resistances  of  the  windings. 
Opposition  test. — Connect  two  identical  trans- 
formers as  in  Fig.  52. 


Supply  the  transformer,  Ts,  with  such  a  voltage  that 
the  ammeter  indicates  that  full-load  current  is  flowing 
in  the  coils  of  the  transformer  under  test.  Take  periodic 
readings  of  the  thermometers  placed  in  the  oil  of  the 
transformer  and  of  the  wattmeters  connected  in  the 
supply  circuits. 

6.  Ultimate  Temperature. — It  is  often  inconvenient  to 
prolong  a  heat  test  until  the  maximum  temperature  is 


reached.  This  temperature  may  be  calculated  in  the 
following  manner :  On  the  heating  curve  (Fig.  54)  mark 
off  four  equal  abscissa  whose  ordinates  are  TI,  T2,  T3,  T4. 
The  ultimate  temperature 

7\ 


1- 


T4-T3 


i4&=3 


9]  ALTERNATING  CURRENTS  99 

T, 

1- 


T  —  T 
1  2      7  i 

(Note. — All  four  calculations  should  be  made  and  the 
average  value  taken  as  the  value  to  which  the  temperature 
of  the  transformer  will  ultimately  rise.) 

7.  Separation  of  the  Iron  Losses. — The  iron  losses  of  a 
transformer  may  be  separated  into  hysteresis  and  eddy- 
current  losses  as  in  Section  4  of  the  experiment  on  Iron 
Losses. 

8.  Obtain  data  for  and  construct 

(a)  iron-loss  curve. 
(6)   copper-loss  curve. 

(c)  efficiency  curve. 

(d)  Kapp's  diagram  for 

(1)  100  per  cent,  power  factor. 

(2)  80  per  cent,  power  factor  leading. 

(3)  80  per  cent,  power  factor  lagging. 

(e)  temperature  curve  from 

(1)  thermometer  readings. 

(2)  resistance. 

9.  Determine 

(a)  per  cent,  regulation  for 

(1)  100  per  cent,  power  factor. 

(2)  80  per  cent.  cent,  power  factor  leading. 

(3)  80  per  cent,  power  factor  lagging. 
(6)  the  equivalent  resistance. 

(c)  the  equivalent  reactance. 

(d)  the  ultimate  temperature. 


100  DYNAMO  LABORATORY  OUTLINES  [9 

10.  Explain 

(a)  the  advantages  of  the  opposition  test. 
(6)  the  action  of  the  transformer,  Ts,  in  the  oppo- 
sition test. 

(c)  why  the  temperature  as  calculated  from  resist- 
ance measurements  differs  from  that  indicated 
by  thermometers. 

(d)  "  all-day  efficiency.7' 

(e)  equivalent  resistance  and  reactance. 

(/)  why  the  determination  of  regulation  by  loading 
is  usually  unsatisfactory. 

(g)  why  transformers  should  be  rated  in  KVA  in- 
stead of  in  KW. 

(h)  the  relation  of  the  losses  at  maximum  efficiency. 

(i)  the  disadvantages  of  an  efficiency  test  by  load- 
ing. 

(k)  why  the  iron  losses  change  as  the  temperature  of 
the  transformer  increases. 

(I)  the  effect  of  the  power  factor  of  the  load  circuit 
on  the  regulation  of  a  transformer. 

11.  Show 

(a)  that  the  copper  losses  are  negligible  in  the  meas- 
urements for  the  determination  of  iron  losses  and 
that  the  iron  losses  are  negligible  in  the  meas- 
urements for  the  determination  of  copper  losses. 

(6)  that  a  resistance  in  the  primary  circuit  and  one 
in  the  secondary  circuit  are  equivalent  when 
their  ratio  is  the  square  of  the  ratio  of  the  number 
of  turns  in  the  windings. 

REFERENCES 

Franklin  &  Esty,  Alternating  Currents,  Chap.  10-11. 
Karapetoff,  Exp.  Elec.  Eng.,  Chap.  19. 
Bedell,  D.-C.  &  A.-C.  Testing,  Chap.  5. 


10] 


ALTERNATING  CURRENTS 


Fleming,  The  Transformer. 

Smith,  Alternating  Currents,  Chap.  6. 

Steinmetz,  Elements,  pp.  60-79. 

Foster's  Handbook. 

Standard  Handbook. 


101 


10 


THE  AUTO-TRANSFORMER 

The  object  of  this  experiment  is  to  study  the  single-coil 
or  auto-transformer. 

1.  Connect  an  auto-transformer  as  in  Fig.  55,  and  read 
watts,  volts  and  amperes  for  various  loads. 


FIG.  55. 


2.  Calculate 


(a)  efficiency. 
(6)  regulation. 

3.  Explain 

(a)  the  current  relations  as  indicated  by  the  three 

ammeters. 
(6)  the  voltage  relations. 

(c)  why  it  is  not  advisable  to  use  auto-transformers 
for  lighting  or  power  service. 

(d)  the  use  of  an  ordinary  10  : 1  transformer  as  an 
auto-transformer. 


102  DYNAMO  LABORATORY  OUTLINES  [11 

4.  Compare 

(a)  the  copper  losses  in  the  two  parts  of  the  winding 
of  -an  auto-transformer. 

(6)  "the  total  'copper  loss  of  an  auto-transformer  with 
that  of  'an  ordinary  transformer  having  the 
same  primary  and  secondary  e.m.f.,  the  same 
primary  and  secondary  resistances  and  the  same 
output. 

5.  Give  the  chief  commercial  uses  of  the  auto-trans- 
former. 

REFERENCES 

Franklin  &  Esty,  Alternating  Currents,  pp.  219-221. 
Karapetoff,  Exp.  Elec.  Eng.,  Vol.  1,  pp.  342-343. 
Bedell,  D.-C.  &  A.-C.  Testing,  Chap.  5. 
Smith,  Alternating  Currents,  Chap.  6. 
Standard  Handbook. 
Foster's  Handbook. 

11 

TRANSFORMER  CONNECTIONS 

The  object  of  this  experiment  is  to  study  the  various 
transformer  connections,  both  single-phase  and  polyphase 

1.  Single-phase. — A  variety  of  connections  (giving 
different  secondary  voltages)  for  the  single-phase  trans- 
former are  available  when  the  primary  and  the  secondary 
windings  are  divided  into  two  coils,  as  is  the  case  in 
most  commercial  transformers.  The  following  connec- 
tions are  in  more  general  use: 

(a)  Both   primary   and  secondary   coils   in   series. 

Fig.  56. 
(6)  Both  primary  and  secondary  coils  in  parallel. 

Fig.  57. 


11] 


ALTERNATING  CURRENTS 


103 


(c)  Primary  coils  in  series,  secondary  coils  in  parallel. 
Fig.  58. 

(d)  Primary   coils   in   parallel,    secondary   coils   in 
series.     Fig.  59. 

In  (a)  and  (d)  a  three-wire  distributing  system  is 
obtained  by  the  addition  of  a  connection  at  the  junction 
of  the  secondary  coils,  as  indicated  by  the  dotted  lines. 


FIG.  56. 


FIG.  57. 


(Warning. — It  is  possible  to  connect  the  coils  of  a 
transformer  so  that  they  form  a  local  short-circuit. 
Hence,  it  is  always  advisable  in  making  any  connections, 
to  protect  the  transformers  by  circuit  breakers  or  fuses 
of  proper  capacity  in  the  primary  circuit.) 


FIG.  58. 


FIG.  59. 


2.  Polyphase. — Two-phase  currents  are  transformed 
by  connecting  a  single  transformer  to  each  phase,  all 
the  connections  given  under  (1)  being  available.  In 
addition,  interconnection  of  the  primaries  or  of  the 
secondaries,  or  of  both,  may  be  made. 

Three-phase  transformer  connections  are  the  following: 


104  DYNAMO  LABORATORY  OUTLINES  [11 

(a)  Delta-delta. — In  this  connection  three  trans- 
formers are  used,  the  three  coils  (primary  or 
secondary)  forming  the  sides" of  a  triangle,  the  line 
connections  being  made  at  the  corners  of  the 
triangle  or  the  junction  of  two  transformer 
windings.  Fig.  60. 


FIG.  60.  FIG.  61. 

(b)  Star-star. — In  this  connection  three  terminals 
(primary  or  secondary)  are  tied  together,  the 
other  three  terminals  being  connected  to  the 
line.     Fig.  61. 

(c)  Delta-star. — This  connection  is  a  combination 
of  (a)  and  (6),  the  primaries  being  connected 
delta  and  the  secondaries  star.     Fig.  62. 


FIG.  62.  FIG.  63. 

(d)  Star-delta. — This  connection  is  the  same  as  (c) 
except  that  the  primaries  are  star  and  the 
secondaries  delta. 

(Warning. — The  same  precautions  should  be  taken 
to  protect  the  transformers  when  making  polyphase 
connections  as  for  single-phase,  and  tests  made  to  see 
that  the  triangle  of  voltages  is  symmetrical.) 


11] 


ALTERNATIMG  CURRENTS 


105 


3.  V-  or  Open-delta. — It  is  possible  to  operate  a 
three-phase  system  with  only  two  transformers  although 
the  current  relations  are  somewhat  distorted.  The 
connections  are  shown  in  Fig.  63. 


4.  For  the  operation  of  large  rotary  converters,  it  is 
desirable  to  use  six  phases.  Three-phase  to  six-phase 
transformation  may  be  accomplished  in  any  of  the  follow- 


FIG.  65. 

ing  ways,  the  primaries  being  connected  either  star  or 
delta  to  three-phase  mains : 

(a)  Double-delta. — This  connection  requires  the 
use  of  transformers,  the  secondary  windings 
of  which  are  divided.  By  means  of  the  con- 


106 


DYNAMO  LABORATORY  OUTLINES 


[11 


(6) 


nections  indicated  in  Fig.  64,  two  deltas  are 
formed   which,   when  taken  together,   form   a 
six-phase  system. 
Double-star. — In  this  connection  one  terminal 


_QQfiQQfiQ&L 

_QOOOQQPOQ_ 

J&Q&H&flfl. 

"QfflftRRftflfiT 

~&WOOOOO(T~ 

IRRJMoood" 

A 

B 

c 

1                      £ 

! 

i 

FIG.  66. 


of  each  of  the  six  secondary  coils  is  connected 
to  a  common  point,  the  remaining  terminals 
forming  the  line  connections.     Fig.  65. 
(c)    Diametral. — In  this   connection  the  terminals 


000000 


OOQOOO 


FIG.  67. 


of  transformer  A  are  connected  to  rings  (1)  and 
(4)  of  the  rotary,  those  of  B  to  rings  (2)  and  (5) 
and  those  of  C  to  rings  (3)  and  (6).     Fig.  66. 
(d)  Hexagonal. — The  six  secondary  coils  are  joined 


11] 


ALTERNATING  CURRENTS 


107 


so  as  to  form  a  hexagon,  a  line  connection  being 
made  at  the  junction  of  two  coils.     Fig.  67. 

5.  The  Scott  Transformation. — Two-phase  to  three- 
phase  or  three-phase  to  two-phase  transformation  is 
accomplished  by  means  of  two  transformers  connected 
as  in  Fig.  68.  The  three-phase  coils  are  connected  in 
"T, "  i.e.,  the  terminal  of  one  coil  is  connected  to  the 
middle  point  of  the  other  coil.  The  two-phase  coils 
are  independent  of  each  other.  To  get  the  proper 


OOOOOOOOOOQO 


000000000 


FIG.  68. 


voltage  relations  it  is  necessary  that  the  number  of 
turns  in  the  three-phase  coil  of  transformer  B  equal 
0.866  times  the  number  of  turns  in  the  three-phase  coil 
of  transformer  A. 

6.  Find    the    voltage    relations    when    single  10 :  1 
transformers  are  connected 

(a)  as  in  Fig.  56. 

(6)  as  in  Fig.  57. 

(c)  as  in  Fig.  58. 

(d)  as  in  Fig.  59. 

(e)  as  in  Fig.  60. 
(/)  as  in  Fig.  61. 
(0)  as  in  Fig.  62. 
(h)  as  in  Fig.  63. 


108  DYNAMO  LABORATORY  OUTLINES  [12 

(i)  as  in  Fig.  64. 
(fc)  as  in  Fig.  65. 
(I)  as  in  Fig.  66. 
(m)  as  in  Fig.  67. 

7.  Find  the  two-phase  voltage  when  two  10 :  1  trans- 
formers are  used  to  transform  2,200-volt  three-phase  to 
two-phase  by  Scott's  method. 

8.  Explain 

(a)  why  the  measured  voltage  from  line  to  neutral 
in  a  three-phase  star-delta  connection  may  not 
be  equal  to  the  line  to  line  voltage  divided  by 
the  square  root  of  three. 

(6)  by  means  of  a  clock  diagram,  the  three-phase 
current  and  voltage  relations  in  a  Scott 
transformation. 

(c)  by  means  of  a  clock  diagram,  the  current  and 
voltage  relations  in  the  V-or  open-delta  con- 
nection. 

(d)  the  advantages  of  and  the  objections  to  the 
V-connection. 

(e)  the   advantages  of  star  connection  and  state 
where    these    advantages    become    highly    im- 
portant. 

(/)    the  advantage  of  delta  connection  for  secondary 

distributing  systems. 
(g)  the  use  of  the  star-delta  system. 

REFERENCES 

Franklin  &  Esty,  Alternating  Currents,  Chap.  10. 
Karapetoff,  Exp.  Elec.  Eng.,  Chap.  20. 
Steinmetz,  A.-C.  Phenomena,  Chap.  36. 
Smith,  Alternating  Currents,  Chap.  6! 
Standard  Handbook. 
Foster's  Handbook. 


12] 


ALTERNATING  CURRENTS 


109 


12 

IRON  LOSSES 

The  object  of  this  experiment  is  to  determine  the  losses 
in  iron,  to  separate  such  losses  into  their  components 
(hysteresis  and  eddy  current)  and  to  determine  the  value 
of  Steinmetz's  exponent  and  of  Steinmetz's  coefficient. 

1.  Apparatus. — The  apparatus  used  in  this  experiment 
is  that  of  Eppstein,  and  consists  of  four  coils  of  insulated 


FIG.  69. 

wire  connected  in  series  and  arranged  to  form  the  sides 
of  a  square  as  in  Fig.  69.  The  cores  of  the  coils  are  built 
up  of  strips  of  the  iron  to  be  tested,  the  strips  being  of 
uniform  dimensions  and  arranged  as  shown  (with  "butt" 
joints  at  the  corners). 

It  is  desirable  to  have  the  dimensions  of  the  apparatus 
and  of  the  core  such  that  a  simple  change  of  the  decimal 
point  will  give  the  loss  per  unit  of  weight  and  that  the 
value  of  /?  (maximum  flux  density  for  a  sine  wave  of 
e.m.f.)  may  be  readily  computed  from  the  voltage  and 
the  frequency  of  the  supply  circuit. 

One  of  these  results  is  obtained  by  making  the  total 
weight  of  the  core  10  kilograms  (slightly  greater  than  22 
pounds),  the  other  by  making  the  area  of  the  core  and 
the  number  of  turns  in  the  coils  such  that 


110  DYNAMO  LABORATORY  OUTLINES  [12 

108 


4.44NA 


=  k 


when    k  =  a  constant  the  value  of  which  is  some  multiple 

of  100. 

N  =  the  number  of  series  turns  on  the  coils. 
A  =the  cross  sectional  area  of  the  iron. 

Then 


when  E  =  the  effective  value  of  the  e.m.f.  induced  in 
the  coil  but  which  may  be  taken,  in  a  properly  con- 
structed apparatus,  as  equal  to  the  e.m.f.  impressed  on 
the  terminals  of  the  coils. 

/=  frequency  of  the  supply  circuit. 

2.  Connect  the  apparatus  as  shown  in  Fig.  69  and  take 

readings  over  as  wide  a  range  of  voltage  as  possible,  the 

E 
frequency  being  varied  so  that  -j  is  a  constant,  thus  keep- 

ing the  flux  density  constant. 

3.  Repeat    (2)    for   several   different    values    of   flux 
density. 

4.  The  wattmeter  will  indicate  the  iron  loss  plus  a 
small  copper  loss.     In  a  properly  constructed  apparatus 
the  latter  loss  is  negligible. 

Then 


when  k 

and  k 

By  dividing  the   above  expression  by  /  we  obtain 


12] 


ALTERNATING  CURRENTS 


111 


W 
which  is  the  equation  of  a  straight  line  between  y-  and  /. 

Plot  this  curve  (ab  Fig.  70)  and  extend  it  to  the  inter- 
section with  the  axis  of  ordinates.  The  value  of  the 
ordinate,  oc,  is  that  of  the  constant,  kh. 

By  multiplying  the  ordinate,  oc  =  kh,  by  any  frequency, 
the  watts  lost  in  hysteresis  is  determined  for  that  fre- 
quency and  the  flux  density  (constant)  for  which  the 
measurements  were  taken. 


Frequency 

FIG.  70. 

Likewise  the  eddy-current  loss  is  obtained  for  this  flux 
density  and  any  given  frequency  by  multiplying  the  or- 
dinate, kef,  for  that  frequency  by  the  frequency. 

The  hysteresis  and  eddy-current  losses  determined 
above  may  be  plotted  as  in  Fig.  71. 

5.  Steinmetz's  Exponent. — The  equation  for  hysteresis 
loss 


may  be  written 


112 


DYNAMO  LABORATORY  OUTLINES 


[12 


since  a,  f  and  V  are  constants,  and  /?  is  proportional  to 
E.     The  latter  expression  may,  in  turn,  be  written 

log  W  =  log  khl+x  log  E. 


Eddy  Currents 


Hysteresis 


Frequency 

FIG.  71. 


This  equation  is  that  of  a  straight  line  between  log 
W  and  log  E,  the  tangent  of  the  angle  between  the  line 
and  the  axis  of  abscissa  being  the  required  value  of  x. 


Log  E 

FIG.  72. 


(Note. — For  the  usual  range  of  flux  densities  this  angle 
should  approximate  58°  for  which  the  value  of  #  =  1.6, 


12]  ALTERNATING  CURRENTS  113 

which  is  the  value  commonly  assigned  for  Steinmetz's 
exponent.) 

Find,  by  means  outlined  above,  the  hysteresis  loss  for 
several  values  of  E  (over  as  wide  a  range  as  possible) 
but  for  the  same  frequency,  so  that  the  flux  density  will 
change,  plot  the  log  equation  as  in  Fig.  72  and  determine 
the  value  of  x. 

6.  Steinmetz's  Coefficient.—  If  the  hysteresis  loss  in 
iron  is  expressed  in  ergs  per  cubic  centimeter  per  cycle, 
we  have 


N  being  Steinmetz's  coefficient  which  expresses  the  mag- 
netic  quality   of  the   iron.     From   the   hysteresis   loss 

determined  above,  the  dimensions  of  the  core  and  the 

pi 

constant  of  the   apparatus  (P  =  k-j)  the  quality  of  the 

iron  is  determined. 

7.  Determine 

(a)  hysteresis  and  eddy-current  losses  and  plot  them 

as  in  Fig.  71. 

(6)  the  value  of  Steinmetz's  exponent. 
(c)   the  electrical  quality  of  the  iron. 

8.  Explain 

(a)  the  effect  of  a  change  of  frequency  on  the  iron 

losses. 
(6)   how  varying  the  voltage  and  the  frequency  in 

the  same  ratio  keeps  the  flux  density  constant. 
(c)   the  effect  on  the  iron  losses  of  laminating  the 

iron. 

9.  If  the  iron  tested  were  used  in  the  core  of  a  2,200  :  220- 
volt  transformer,  determine  the  iron  losses  at  60  cycles 
as  compared  to  those  at  25  cycles. 


114  DYNAMO  LABOKATORY  OUTLINES 


[13 


REFERENCES 

Franklin  &  Esty,  Alternating  Currents,  pp.  211-212. 
Steinmetz,  A.-C.  Phenomena,  pp.  169-216. 
Karapetoff,  Exp.  Elec.  Eng.,  Chap.  10. 
Karapetoff,  The  Magnetic  Circuit,  Chap.  3. 
Smith,  Alternating  Currents,  pp.  160-163,  229-233. 
Standard  Handbook. 
Foster's  Handbook. 


13 

THE  CONSTANT  CURRENT  TRANSFORMER 

The  object  of  this  experiment  is  to  study  the  trans- 
former used  to  supply  constant  current  to  a  series  arc- 
lighting  system  from  constant  potential  mains. 


FIG.  73. 

1.  Connect  as  in  Fig.  73  and  record  the  indications  of 
the  instruments  for  an  increasing  number  of  lamps. 

2.  Repeat  (1)  for  a  non-inductive  load,  such  as  a  water 
rheostat. 

3.  Determine  the  resistance  of  the  primary  and  of  the 


13]  ALTERNATING  CURRENTS  115 

secondary  winding,  from  which  the  copper  losses  for  any 
current  value  may  be  calculated. 

Then 
Iron  losses  =  watts  input  — watts  output  —  copper  losses. 

4.  Plot  curves  using  KW  output  as  abscissa  and  the 
following  as  ordiriates: 

(a)  primary  e.m.f. 

(6)  primary  current.  "  * 

(c)  primary  power  factor. 

(d)  secondary  e.m.f. 

(e)  secondary  current. 

(/)  secondary  power  factor. 

(</)  total  copper  losses. 

(h)  iron  losses. 

(i)  efficiency. 

5.  Explain 

(a)  how  the   primary   current   and   e.m.f.   remain 

practically  constant  with  varying  load. 
(6)  why  the  iron  losses  change  as  the  load  varies. 

(c)  the  relation  between  the  losses  at  maximum 
efficiency. 

(d)  the  cause  of  the  low  power  factor  of  the  arc-lamp 
circuit. 

(e)  why  the  distance  between  the  two  coils  varies 
as  the  load  changes. 

(/)  how  the  ratio  of  voltage  transformation  changes, 
the  ratio  of  the  number  of  turns  in  the  coils 
being  constant. 

REFERENCES 

Franklin  &  Esty,  Alternating  Currents,  pp.  215-217. 
Karapetoff,  Exp.  Elec.  Eng.,  Vol.  1,  pp.  240-243. 
Crocker,  Electric  Lighting,  pp.  171-174. 
Foster's  Handbook. 
Standard  Handbook. 


116 


DYNAMO  LABORATORY  OUTLINES 


[14 


14 

THE  MERCURY-ARC  RECTIFIER 

The  object  of  this  experiment  is  the  study  of  the 
mercury  arc  as  used  for  the  rectification  of  alternating 
currents. 

1.  The  mercury-arc  rectifier  consists  of  a  vacuum  tube 
containing  a  small  quantity  of  mercury,  which  forms  an 


Single  Phase 


Three  Phase 


-Starting 
Switch 


D.C.Circuit 

FIG.  74. 


electrode  to  which  one  terminal  of  the  D.-C.  circuit  is 
connected,  two  or  more  iron  or  graphite  terminals,  to 
which  the  A.-C.  lines  are  connected  and  an  auxiliary 
mercury  electrode  for  starting.  It  may  be  operated  on 
either  single-phase  or  polyphase  circuits.  Diagrams 
for  single-  and  for  three-phase  circuits  are  shown  in  Fig. 


14]  ALTERNATING  CURRENTS  117 

74.  The  direct  current  is  constant  potential  or  con- 
stant amperage,  depending  on  whether  T  is  a  constant 
potential  or  a  constant  current  transformer. 

2.  Starting. — Close  the  D.-C.  circuit  (through  a  resist- 
ance such  that  the  current  will  not  become  excessive) , 
close  the  starting  switch  and  shake  or  tilt  the  bulb  until 
the  two  mercury  electrodes  are  connected  by  the  liquid. 

Open  the  starting  switch   and  let  the  bulb   return 
to  its  normal  position. 

3.  Measure  the  following  for  constant  A.-C.  voltage: 

(a)  A.-C.  voltage. 
(6)   A.-C.  amperes. 

(c)  'D.-C.  voltage. 

(d)  D.-C.  amperes. 

(e)  watts  input. 
(/)    watts  output. 

4.  Repeat  (3)  for  constant  current  in  the  D.-C.  circuit. 

5.  Calculate  the  following  for  (3)  and  (4) : 

(a)  efficiency. 

(6)   power  factor  of  the  A.-C.  circuit. 

6.  With  watts  output  as  abscissa,  plot  the,  following 
curves  from  data  obtained  in  (3)  and  (4) : 

(a)  A.-C.  voltage. 
(6)  A.-C.  amperes. 

(c)  D.-C.  voltage. 

(d)  D.-C.  amperes. 

(e)  efficiency. 

(/)   power  factor  of  the  A.-C.  circuit. 

7.  Explain 

(a)  the  need  of  the  auxiliary  electrode  and  starting 

circuit. 
(6)  the   use   and   effect   of   an  inductance   in  the 

D.-C.  circuit. 


118  DYNAMO  LABORATORY  OUTLINES  [15 

(c)  why  the  apparatus  will  not  operate  with  the 
D.-C.  circuit  open. 

(d)  why    the    efficiency    increases    with    increased 
A.-C.  voltage. 

(e)  why  A.-C.  instruments  should  be  used  in  the 
D.-C.  circuit. 

(/)    the  conditions  affecting  the  life  of  the  tube  and 

how  the  life  is  prolonged. 
(g)  why  the  mercury-arc   rectifier   is  not  used  to 

supply  D.-C.  motors. 
(h)  why  the  arc  "breaks''  when  the  D.-C.  amperes 

fall  below  a  certain  minimum  value. 
(i)    the  effect  should  mercury  be  deposited  on  the 

alternating-current  terminals. 
(k)  the  relation  of  the  D.-C.  voltage  to  the  A.-C. 

voltage. 

REFERENCES 

Franklin  &  Esty,  Alternating  Currents,  pp.  171-172. 
Standard  Handbook. 
Foster's  Handbook. 
Current  publications. 

15 

INSULATION  (BREAKDOWN)  TEST 

The  object  of  this  experiment  is  to  familiarize  the 
student  with  the  conduct  of  a  high-voltage,  insulation 
test  and  to  determine  the  relative  insulating  values  of 
different  materials. 

1.  Connect  a  high-voltage  transformer  as  in  Fig.  75. 
Supply  the  primary  (low  voltage)  of  the  transformer 
from  a  circuit  the  voltage  of  which  may  be  varied 
between  wide  limits. 

(Warning. — Extreme  care  must  be  taken  during  the 


15] 


ALTERNATING  CURRENTS 


119 


performance  of  this  experiment  that  no  part  of  the 
apparatus  is  touched  except  when  the  switch  of  the 
primary  circuit  is  open,  as  the  voltage  is  likely  to  be 
very  high.) 

2.  Flat    Insulation. — Materials    like    Fuller    board, 
cloth  and  mica  may  be  tested  by  placing  a  suitable 


—  ™ 

1    -1 

Insulation    '  fSpark  ° 

QtH 

Supply 

FIG.  75. 

piece  of  the  material  between  terminals  connected  to 
the  high-voltage  winding,  as  shown  in  Fig.  76,  and 
gradually  increasing  the  applied  voltage  until  break- 
down occurs. 

Separate  the  terminals  and  determine,  by  means  of  the 

Insulation 

Terminal  Plates 


FIG.  76. 

spark  gap,  the  " sparking"  distance,  in  air,  for  the  same 
primary  (applied)  voltage.  The  breakdown  voltage  may 
be  determined  by  reference  to  Fig.  77. 

(Note. — Do  not  use  burned  or  blunt  needles  in  deter- 
mining the  sparking  distance.) 


120  DYNAMO  LABORATORY  OUTLINES 


[15 


Inches 
26 24       22       20       18       16      14 


270  ^ 


240 


210 


180 


1150 


120 


20 


30 


2        4        6        8       10       12      14 
Inches 


15] 


ALTERNATING  CURRENTS 


121 


3.  Wire  Insulation. — Immerse  the  wire,  the  insulation 
of  which  is  to  be  tested,  in  a  vessel  of  salt  water,  connect 
one  terminal  of  the  transformer  to  the  salt  water,  the 
other  to  the  wire  and  proceed  as  in  (2). 

4.  Porcelain,  Glass  and  Other  Shaped  Insulation.— 
Connect  to  the  high-voltage  winding  by  means  of  salt- 
water terminals  and  proceed  as  in  (2). 


FIG.  78. 

Darken  the  room  and  note 

(a)  the  discharge  over  the  surface  of  a  high-voltage 

pole-line  insulator. 
(6)    the  corona  at  high  voltages. 

5.  Oil/ — When  oil  is  to  be  tested  it  is  placed  in  a  glass 
vessel  having  two  polished  spherical  terminals  (Fig.  78) 
which  are  connected  to  the  high-voltage  winding  of  the 
testing  transformer.     The  upper  terminal  is   movable 
by  means  of  a  micrometer  screw  or  other  device,  so  that 
the  distance  between  the  balls  may  be  easily  changed  and 
measured. 

6.  Compare  the  insulating  properties  of 

(a)  different  materials  of  the  same  thickness. 


122 


DYNAMO  LABORATORY  OUTLINES 


[16 


(6)  different  thicknesses  of  the  same  material. 

(c)   two  or  more  layers  of  a  given  material  with  a 

single  layer  of  the  same  total  depth. 
7.  Determine  the  breakdown  voltage  of 
(a)  wire  or  cable  insulation. 
(6)  transformer  oil. 

REFERENCES 

Karapetoff,  Exp.  Elec.  Eng.,  Vol.  2,  pp.  82-88. 
Standard  Handbook. 
Foster's  Handbook. 


16 

WAVE  FORM 

The  object  of  this  experiment  is  to  study  the  form  of 
e.m.f.  and  current  waves  and  to  determine  the  effects  of 
different  types  of  load  on  the  wave  form. 


FIG.  79. 

1.  Two  methods  are  available  for  determining  the 
shape  of  an  e.m.f.  or  current  wave,  (a)  the  point-to-point 
method,  (6)  the  oscillograph. 

(a)  The  contact  maker. — A  simple  and  easily 
applied  point-to-point  method  consists  of  the 
connections  shown  -in  Fig.  79,  where  G  is  the 
generator,  the  wave  form  of  which  is  to  be 


16] 


ALTERNATING  CURRENTS 


123 


(6) 


determined,  or  a  small  synchronous  motor 
supplied  with  current  from  such  a  source.  C  is 
a  contact  maker  driven  by  the  generator  (or  the 
motor)  and  so  arranged  that  an  instantaneous 
current  is  sent  through  the  circuit  once  in  each 
revolution. 

The  oscillograph. — In  the  oscillograph,  rotating 
or  vibrating  mirrors  are  caused  to  reflect  a  beam 
of  light  onto  a  screen  or  a  photographic  plate. 
Two  motions,  at  right  angles  to  each  o'ther,  are 


Shield  Winding 

for 
oElectro-magnet 


FIG.  80. 

required  to  give  a  correct  representation  of  wave 
form,  one  proportional  to  the  magnitude  of  the 
current  or  e.m.f.,  the  other  constant.  Fig.  80 
represents  the  electrical  circuits  of  an  oscillo- 
graph provided  with  two  vibrators  so  that 
the  current  and  e.m.f.  waves  may  be  taken 
simultaneously. 

M  is  a  small  synchronous  motor  which  vibrates  or 
rotates  a  mirror,  the  period  of  vibration  of  which  is 
constant;  vi  and  vz  are  the  vibrators  the  deflections  of 
which  are  proportional  to  the  instantaneous  value  of  the 
current  or  the  e.m.f.  R  is  a  current  shunt  in  series  with 


124  DYNAMO  LABORATORY  OUTLINES  [16 

the  load  while  Ri  and  Rz  are  resistances  in  the  vibrator 
circuits. 

3.  Determine  the  current  and  the  e.m.f.  wave  from  for 

(a)  non-inductive  load, 

(b)  over-excited  synchronous  motor. 

(c)  under-excited  synchronous  motor. 

(d)  synchronous  motor  load  at  unity  power  factor. 

(e)  induction  motor  load. 

(/)    an  unloaded  transformer. 

(g)  "the  primary  of  a  constant  current  transformer. 

(h)  the  secondary  of  a  constant  current  transformer 
(1)  on  an  arc  circuit,  (2)  on  non-inductive  circuit. 

(k)  the  secondary  of  a  constant  current  transformer 
(1)  on  an  arc  circuit,  (2)  on  non-inductive 
circuit. 

(i)    the  mercury  arc  rectifier  (A.-C.  side). 

(k)  the  mercury  arc  rectifier  (D.-C.  side). 

(I)  current  in  neutral  between  generator  and  a  three- 
phase  motor;  e.m.f.  line  to  neutral. 

(m)  current  in  neutral  between  star-connected 
transformers;  e.m.f.  line  to  neutral. 

(ri)  current  in  line;  e.m.f.  between  lines  of  a  three- 
phase,  non-inductive  system. 

(o)   open-delta  transformer  connection. 

(p)  three-phase  and  two-phase  sides  of  a  Scott 
transformation.  (Test  the  current  and  the 
voltage  relations  in  each  line  and  the  junction 
of  the  coils  on  the  three-phase  side.)  . 

4.  Explain 

(a)  the  use  of  the  condenser  in  the  contact  method. 

(b)  why  the   waves   differ   in   shape   for   different 
loads. 

(c)  the  current  and  voltage  relations  found  in  (3p). 

(d)  how   the   power   factor   of   a   circuit   may   be 
determined  from  an  oscillogram. 


16]  ALTERNATING  CURRENTS  125 

(e)   the  cause  of  the  triple  frequency  currents  in  the 
neutral  of  a  star-connected  system. 

REFERENCES 

Karapetoff,  Exp.  Elec.  Eng.,  Chap.  27. 
Kinzbrunner,  Alternating  Currents,  Chap.  8. 


APPENDIX 

POWER  MEASUREMENTS  AND  WATTMETER  CONNECTIONS 

This  article  is  written  for  reference  in  making  connec- 
tions for  the  measurement  of  power  in  the  above  experi- 
ments rather  than  as  an  additional  experiment.  It  is 
the  writer's  belief  that  all  the  experimental  work 
desirable  in  power  measurement  can  be  best  given  in 
connection  with  other  experiments  where  the  measure- 
ment of  power  is  required,  by  the  use  of  different  watt- 
meter connections  for  different  experiments. 

1.  General  Method. — The  general  method,  which 
requires  one  less  wattmeter  than  there  are  conductors  in 
the  system,  is  applicable  to  any  system  of  any  number  of 
phases  and  under  any  condition  of  balanced  or  unbalanced 
loading.  Any  conductor  may  be  regarded  as  a  common 
return  for  all  the  others.  The  current  coil  of  a  watt- 
meter is  connected  in  each  conductor  (except  the  one 
selected  as  the  common  return)  and  the  potential  coil  is 
connected  from  the  conductor  to  the  common  return. 
The  algebraic  sum  of  the  wattmeter  readings  is  the  power 
of  the  system.  The  connections  for  single-phase  (Fig. 
81)  and  for  three-phase  (Fig.  82)  illustrate  this  method. 

In  using  the  general  method  for  three-phase  systems, 
the  readings  are  equal  only  when  the  load  is  balanced  and 
the  power  factor  is  unity.  Thus  in  Fig.  82  the  readings 
will  be  unequal  for  any  load,  balanced  or  unbalanced, 
whose  power  factor  is  less  than  unity  and,  if  the  power 
factor  is  less  than  50  per  cent.,  one  of  the  meter  readings 
is  negative,  i.e.,  the  numerical  difference  of  the  readings 
is  the  power  in  the  circuit. 

126 


APPENDIX  127 

To  determine  if  the  power  factor  of  a  three-phase  load 
circuit  is  greater  or  less  than  50  per  cent.,  measure  the 
power  in  a  non-inductive  circuit,  connecting  the  coils  so 
that  the  meters  indicate  properly.  With  the  same  meter 
connections  measure  the  power  in  the  circuit  whose 
power  factor  is  unknown  and  if  the  terminals  of  the 


FIG.  81. 

current  coil  of  one  wattmeter  must  be  reserved  to  make 
it  indicate  properly,  the  reading  of  that  meter  should  be 
regarded  as  negative  and  the  power  factor  of  the  circuit  is 
less  than  50  per  cent. 

Another  simple  method  of  determining  whether  the 
power  factor  of  the  load  circuit  is  greater  or  less  than  50 


FIG.  82. 

per  cent,  is  to  interchange  the  two  meters  without  altering 
the  relative  connections  of  the  current  and  potential 
coils.  If  the  deflections  are  in  the  same  direction  as 
before  the  meters  were  interchanged,  the  power  factor  is 
greater  than  50  per  cent. 

From  the  two  wattmeter  readings  the  angle  between 


128  DYNAMO  LABORATORY  OUTLINES 

the  current  and  the  e.m.f.  vectors  may  be  determined, 
for  balanced  load  and  sine  waves,  from  the  following 
equation  : 

j^7  —W 


By  providing  suitable  switches  so  that  one  meter 
may  be  alternately  connected  to  different  phases,  only 
one  meter  is  required  for  the  measurement  of  power  by 
the  two-wattmeter  method. 

2.  Single  Meter  Methods  on  Three-phase  Systems.— 
When  the  potential  coil  of  the  wattmeter  can  be  con- 
nected to  the  neutral  of  a  three-phase  system,  the  con- 


FIG.  83. 

nections  shown  in  Fig.  83  may  be  used.  If  the  load  is 
balanced,  three  times  the  wattmeter  indication  is  the 
total  power  of  the  system  or  the  meter  may  be  calibrated 
to  read  total  power  direct.  If  the  load  is  not  balanced, 
switches  must  be  provided  for  connecting  the  current 
coil  successively  in  each  of  the  circuits,  and  the  sum  of 
the  indications  taken  as  the  power  of  the  system. 

Unless  the  ratio  of  the  resistance  of  the  potential 
circuit  of  the  wattmeter  to  Ri,  Rz  and  Rs  is  high,  an 
appreciable  error  is  introduced  due  to  the  shunting  of 
current  around  the  load  resistance.  This  error  is 
eliminated  by  the  use  of  a  "  Y"  multiplier  in  which  two 


APPENDIX 


129 


resistances,  each  equal  to  that  of  the  potential  circuit 
of  the  wattmeter,  are  connected  with  the  wattmeter 
coil  to  form  an  "  artificial  Y."  Fig  84. 

3.  Polyphase  Wattmeters.  —  The  polyphase  watt- 
meter is  a  combination  of  two  or  more  single  elements 
acting  on  the  same  moving  part.  Each  pair  of  coils 


FIG.  84. 

(voltage  and  current)  is  connected  as  in  a  single-phase 
meter  and  tested  to  give  a  deflection  in  the  proper 
direction,  considering  the  power  factor  of  the  load  circuit 
as  explained  in  Section  1. 


REFERENCES 

Kinzbrtmner,  Alternating  Currents,  Chap.  6. 
Bedell,  D.-C.  &  A.-C.  Testing,  Chap.  6. 
Karapetoff,  Exp.  Elec.  Eng.,  Chap.  25. 
Smith,  Alternating  Currents,  pp.  278-285. 
Foster's  Handbook. 
Standard  Handbook. 


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